diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 17be72f71a8cb6..603d9c5e22611e 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -883,9 +883,9 @@ strio_rewind(VALUE self)
  * call-seq:
  *   seek(offset, whence = SEEK_SET) -> 0
  *
- * Sets the current position to the given integer +offset+ (in bytes),
+ * Sets the position to the given integer +offset+ (in bytes),
  * with respect to a given constant +whence+;
- * see {Position}[rdoc-ref:IO@Position].
+ * see {IO#seek}[rdoc-ref:IO#seek].
  */
 static VALUE
 strio_seek(int argc, VALUE *argv, VALUE self)
@@ -1473,170 +1473,8 @@ strio_readline(int argc, VALUE *argv, VALUE self)
  *   each_line(limit, chomp: false) {|line| ... }      -> self
  *   each_line(sep, limit, chomp: false) {|line| ... } -> self
  *
- * With a block given calls the block with each remaining line (see "Position" below) in the stream;
- * returns `self`.
- *
- * Leaves stream position as end-of-stream.
- *
- * **No Arguments**
- *
- * With no arguments given,
- * reads lines using the default record separator global variable `$/`, whose initial value is `"\n"`.
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line {|line| p line }
- * strio.eof? # => true
- * ```
- *
- * Output:
- *
- * ```
- * "First line\n"
- * "Second line\n"
- * "\n"
- * "Fourth line\n"
- * "Fifth line\n"
- * ```
- *
- * **Argument `sep`**
- *
- * With only string argument `sep` given,
- * reads lines using that string as the record separator:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(' ') {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First "
- * "line\nSecond "
- * "line\n\nFourth "
- * "line\nFifth "
- * "line\n"
- * ```
- *
- * **Argument `limit`**
- *
- * With only integer argument `limit` given,
- * reads lines using the default record separator global variable `$/`, whose initial value is `"\n"`;
- * also limits the size (in characters) of each line to the given limit:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(10) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First line"
- * "\n"
- * "Second lin"
- * "e\n"
- * "\n"
- * "Fourth lin"
- * "e\n"
- * "Fifth line"
- * "\n"
- * ```
- * **Arguments `sep` and `limit`**
- *
- * With arguments `sep` and `limit` both given,
- * honors both:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(' ', 10) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First "
- * "line\nSecon"
- * "d "
- * "line\n\nFour"
- * "th "
- * "line\nFifth"
- * " "
- * "line\n"
- * ```
- *
- * **Position**
+ * :include: stringio/each_line.md
  *
- * As stated above, method `each` _remaining_ line in the stream.
- *
- * In the examples above each `strio` object starts with its position at beginning-of-stream;
- * but in other cases the position may be anywhere (see StringIO#pos):
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.pos = 30 # Set stream position to character 30.
- * strio.each_line {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * " line\n"
- * "Fifth line\n"
- * ```
- *
- * **Special Record Separators**
- *
- * Like some methds in class `IO`, StringIO.each honors two special record separators;
- * see {Special Line Separators}[rdoc-ref:IO@Special+Line+Separator+Values].
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line('') {|line| p line } # Read as paragraphs (separated by blank lines).
- * ```
- *
- * Output:
- *
- * ```
- * "First line\nSecond line\n\n"
- * "Fourth line\nFifth line\n"
- * ```
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(nil) {|line| p line } # "Slurp"; read it all.
- * ```
- *
- * Output:
- *
- * ```
- * "First line\nSecond line\n\nFourth line\nFifth line\n"
- * ```
- *
- * **Keyword Argument `chomp`**
- *
- * With keyword argument `chomp` given as `true` (the default is `false`),
- * removes trailing newline (if any) from each line:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(chomp: true) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First line"
- * "Second line"
- * ""
- * "Fourth line"
- * "Fifth line"
- * ```
- *
- * With no block given, returns a new {Enumerator}[rdoc-ref:Enumerator].
- *
- * Related: StringIO.each_byte, StringIO.each_char, StringIO.each_codepoint.
  */
 static VALUE
 strio_each(int argc, VALUE *argv, VALUE self)
@@ -2005,10 +1843,10 @@ strio_syswrite_nonblock(int argc, VALUE *argv, VALUE self)
 
 /*
  * call-seq:
- *   strio.length -> integer
- *   strio.size   -> integer
+ *   size -> integer
+ *
+ * :include: stringio/size.rdoc
  *
- * Returns the size of the buffer string.
  */
 static VALUE
 strio_size(VALUE self)