diff --git a/NEWS.md b/NEWS.md
index f847fe6817821d..047bdeeb3e235f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -182,7 +182,7 @@ The following default gems are updated.
 
 * RubyGems 4.0.0.dev
 * bundler 4.0.0.dev
-* erb 5.1.1
+* erb 5.1.3
 * etc 1.4.6
 * fcntl 1.3.0
 * io-console 0.8.1
diff --git a/doc/string/sum.rdoc b/doc/string/sum.rdoc
index 5de24e6402f692..125611411e34ab 100644
--- a/doc/string/sum.rdoc
+++ b/doc/string/sum.rdoc
@@ -1,4 +1,4 @@
-Returns a basic +n+-bit checksum of the characters in +self+;
+Returns a basic +n+-bit {checksum}[https://en.wikipedia.org/wiki/Checksum] of the characters in +self+;
 the checksum is the sum of the binary value of each byte in +self+,
 modulo <tt>2**n - 1</tt>:
 
@@ -9,3 +9,5 @@ modulo <tt>2**n - 1</tt>:
   'こんにちは'.sum  # => 2582
 
 This is not a particularly strong checksum.
+
+Related: see {Querying}[rdoc-ref:String@Querying].
diff --git a/doc/string/swapcase.rdoc b/doc/string/swapcase.rdoc
new file mode 100644
index 00000000000000..93859ec8237fcc
--- /dev/null
+++ b/doc/string/swapcase.rdoc
@@ -0,0 +1,19 @@
+Returns a string containing the characters in +self+, with cases reversed:
+
+- Each uppercase character is downcased.
+- Each lowercase character is upcased.
+
+Examples:
+
+  'Hello World!'.swapcase # => "hELLO wORLD!"
+  'тест'.swapcase         # => "ТЕСТ"
+
+Some characters (and even character sets) do not have casing:
+
+  '12345'.swapcase    # => "12345"
+  'こんにちは'.swapcase # => "こんにちは"
+
+The casing may be affected by the given +mapping+;
+see {Case Mapping}[rdoc-ref:case_mapping.rdoc].
+
+Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].
diff --git a/doc/stringio/each_byte.rdoc b/doc/stringio/each_byte.rdoc
new file mode 100644
index 00000000000000..65e81c53a7cd0f
--- /dev/null
+++ b/doc/stringio/each_byte.rdoc
@@ -0,0 +1,34 @@
+With a block given, calls the block with each remaining byte in the stream;
+positions the stream at end-of-file;
+returns +self+:
+
+  bytes = []
+  strio = StringIO.new('hello')     #  Five 1-byte characters.
+  strio.each_byte {|byte| bytes.push(byte) }
+  strio.eof? # => true
+  bytes # => [104, 101, 108, 108, 111]
+  bytes = []
+  strio = StringIO.new('тест')      # Four 2-byte characters.
+  strio.each_byte {|byte| bytes.push(byte) }
+  bytes # => [209, 130, 208, 181, 209, 129, 209, 130]
+  bytes = []
+  strio = StringIO.new('こんにちは')  # Five 3-byte characters.
+  strio.each_byte {|byte| bytes.push(byte) }
+  bytes # => [227, 129, 147, 227, 130, 147, 227, 129, 171, 227, 129, 161, 227, 129, 175]
+
+The position in the stream matters:
+
+  bytes = []
+  strio = StringIO.new('こんにちは')
+  strio.getc # => "こ"
+  strio.pos  # => 3  # 3-byte character was read.
+  strio.each_byte {|byte| bytes.push(byte) }
+  bytes      # => [227, 130, 147, 227, 129, 171, 227, 129, 161, 227, 129, 175]
+
+If at end-of-file, does not call the block:
+
+  strio.eof? # => true
+  strio.each_byte {|byte| fail 'Boo!' }
+  strio.eof? # => true
+
+With no block given, returns a new {Enumerator}[rdoc-ref:Enumerator].
diff --git a/doc/stringio/each_char.rdoc b/doc/stringio/each_char.rdoc
new file mode 100644
index 00000000000000..d0b5e4082cc4d3
--- /dev/null
+++ b/doc/stringio/each_char.rdoc
@@ -0,0 +1,34 @@
+With a block given, calls the block with each remaining character in the stream;
+positions the stream at end-of-file;
+returns +self+:
+
+  chars = []
+  strio = StringIO.new('hello')
+  strio.each_char {|char| chars.push(char) }
+  strio.eof? # => true
+  chars      # => ["h", "e", "l", "l", "o"]
+  chars = []
+  strio = StringIO.new('тест')
+  strio.each_char {|char| chars.push(char) }
+  chars      # => ["т", "е", "с", "т"]
+  chars = []
+  strio = StringIO.new('こんにちは')
+  strio.each_char {|char| chars.push(char) }
+  chars      # => ["こ", "ん", "に", "ち", "は"]
+
+Stream position matters:
+
+ chars = []
+ strio = StringIO.new('こんにちは')
+ strio.getc # => "こ"
+ strio.pos  # => 3  # 3-byte character was read.
+ strio.each_char {|char| chars.push(char) }
+ chars      # => ["ん", "に", "ち", "は"]
+
+When at end-of-stream does not call the block:
+
+  strio.eof? # => true
+  strio.each_char {|char| fail 'Boo!' }
+  strio.eof? # => true
+
+With no block given, returns a new {Enumerator}[rdoc-ref:Enumerator].
diff --git a/doc/stringio/each_codepoint.rdoc b/doc/stringio/each_codepoint.rdoc
new file mode 100644
index 00000000000000..ede16de599cf0c
--- /dev/null
+++ b/doc/stringio/each_codepoint.rdoc
@@ -0,0 +1,36 @@
+With a block given, calls the block with each successive codepoint from self;
+sets the position to end-of-stream;
+returns +self+.
+
+Each codepoint is the integer value for a character; returns self:
+
+  codepoints = []
+  strio = StringIO.new('hello')
+  strio.each_codepoint {|codepoint| codepoints.push(codepoint) }
+  strio.eof? # => true
+  codepoints # => [104, 101, 108, 108, 111]
+  codepoints = []
+  strio = StringIO.new('тест')
+  strio.each_codepoint {|codepoint| codepoints.push(codepoint) }
+  codepoints # => [1090, 1077, 1089, 1090]
+  codepoints = []
+  strio = StringIO.new('こんにちは')
+  strio.each_codepoint {|codepoint| codepoints.push(codepoint) }
+  codepoints # => [12371, 12435, 12395, 12385, 12399]
+
+Position in the stream matters:
+
+  codepoints = []
+  strio = StringIO.new('こんにちは')
+  strio.getc # => "こ"
+  strio.pos  # => 3
+  strio.each_codepoint {|codepoint| codepoints.push(codepoint) }
+  codepoints # => [12435, 12395, 12385, 12399]
+
+When at end-of-stream, the block is not called:
+
+  strio.eof? # => true
+  strio.each_codepoint {|codepoint| fail 'Boo!' }
+  strio.eof? # => true
+
+With no block given, returns a new {Enumerator}[rdoc-ref:Enumerator].
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index b6ab8cd6cfdeb7..b96010dfbf3f96 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -936,10 +936,9 @@ strio_get_sync(VALUE self)
  * call-seq:
  *   each_byte {|byte| ... } -> self
  *
- * With a block given, calls the block with each remaining byte in the stream;
- * see {Byte IO}[rdoc-ref:IO@Byte+IO].
+ * :include: stringio/each_byte.rdoc
  *
- * With no block given, returns an enumerator.
+ * Related: StringIO#each_char, StringIO#each_codepoint, StringIO#each_line.
  */
 static VALUE
 strio_each_byte(VALUE self)
@@ -1162,12 +1161,11 @@ strio_readbyte(VALUE self)
 
 /*
  * call-seq:
- *   each_char {|c| ... } -> self
+ *   each_char {|char| ... } -> self
  *
- * With a block given, calls the block with each remaining character in the stream;
- * see {Character IO}[rdoc-ref:IO@Character+IO].
+ * :include: stringio/each_char.rdoc
  *
- * With no block given, returns an enumerator.
+ * Related: StringIO#each_byte, StringIO#each_codepoint, StringIO#each_line.
  */
 static VALUE
 strio_each_char(VALUE self)
@@ -1186,10 +1184,9 @@ strio_each_char(VALUE self)
  * call-seq:
  *   each_codepoint {|codepoint| ... } -> self
  *
- * With a block given, calls the block with each remaining codepoint in the stream;
- * see {Codepoint IO}[rdoc-ref:IO@Codepoint+IO].
+ * :include: stringio/each_codepoint.rdoc
  *
- * With no block given, returns an enumerator.
+ * Related: StringIO#each_byte, StringIO#each_char, StringIO#each_line.
  */
 static VALUE
 strio_each_codepoint(VALUE self)
@@ -1630,7 +1627,7 @@ strio_readline(int argc, VALUE *argv, VALUE self)
  * "Fifth line"
  * ```
  *
- * With no block given, returns a new  {Enumerator}[rdoc-ref:Enumerator].
+ * With no block given, returns a new {Enumerator}[rdoc-ref:Enumerator].
  *
  * Related: StringIO.each_byte, StringIO.each_char, StringIO.each_codepoint.
  */
diff --git a/lib/erb/version.rb b/lib/erb/version.rb
index ceca10731e4ce4..138bf3988206fc 100644
--- a/lib/erb/version.rb
+++ b/lib/erb/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 class ERB
   # The string \ERB version.
-  VERSION = '5.1.1'
+  VERSION = '5.1.3'
 end
diff --git a/string.c b/string.c
index b4585fd3205913..fa6ce7f3ec1d07 100644
--- a/string.c
+++ b/string.c
@@ -6190,7 +6190,7 @@ rb_pat_search(VALUE pat, VALUE str, long pos, int set_backref_str)
  *
  *  Like String#sub, except that:
  *
- *  - Changed are made to +self+, not to copy of +self+.
+ *  - Changes are made to +self+, not to copy of +self+.
  *  - Returns +self+ if any changes are made, +nil+ otherwise.
  *
  *  Related: see {Modifying}[rdoc-ref:String@Modifying].
@@ -8203,20 +8203,12 @@ rb_str_capitalize(int argc, VALUE *argv, VALUE str)
  *  call-seq:
  *    swapcase!(mapping) -> self or nil
  *
- *  Upcases each lowercase character in +self+;
- *  downcases uppercase character;
- *  returns +self+ if any changes were made, +nil+ otherwise:
- *
- *    s = 'Hello World!' # => "Hello World!"
- *    s.swapcase!        # => "hELLO wORLD!"
- *    s                  # => "hELLO wORLD!"
- *    ''.swapcase!       # => nil
+ *  Like String#swapcase, except that:
  *
- *  The casing may be affected by the given +mapping+;
- *  see {Case Mapping}[rdoc-ref:case_mapping.rdoc].
- *
- *  Related: String#swapcase.
+ *  - Changes are made to +self+, not to copy of +self+.
+ *  - Returns +self+ if any changes are made, +nil+ otherwise.
  *
+ *  Related: see {Modifying}[rdoc-ref:String@Modifying].
  */
 
 static VALUE
@@ -8240,19 +8232,9 @@ rb_str_swapcase_bang(int argc, VALUE *argv, VALUE str)
 
 /*
  *  call-seq:
- *    swapcase(mapping) -> string
- *
- *  Returns a string containing the characters in +self+, with cases reversed;
- *  each uppercase character is downcased;
- *  each lowercase character is upcased:
- *
- *     s = 'Hello World!' # => "Hello World!"
- *     s.swapcase         # => "hELLO wORLD!"
- *
- *  The casing may be affected by the given +mapping+;
- *  see {Case Mapping}[rdoc-ref:case_mapping.rdoc].
+ *    swapcase(mapping) -> new_string
  *
- *  Related: String#swapcase!.
+ *  :include: doc/string/swapcase.rdoc
  *
  */