gh-106318: Add example for str.isdecimal() (GH-137559)

(cherry picked from commit 6462322)

Co-authored-by: Adorilson Bezerra <adorilson@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: 2 people

Doc/library/stdtypes.rst

@@ -1937,9 +1937,18 @@ expression support in the :mod:`re` module).
    Return ``True`` if all characters in the string are decimal
    characters and there is at least one character, ``False``
    otherwise. Decimal characters are those that can be used to form
-   numbers in base 10, e.g. U+0660, ARABIC-INDIC DIGIT
+   numbers in base 10, such as U+0660, ARABIC-INDIC DIGIT
    ZERO.  Formally a decimal character is a character in the Unicode
-   General Category "Nd".
+   General Category "Nd". For example:
+
+   .. doctest::
+
+      >>> '0123456789'.isdecimal()
+      True
+      >>> '٠١٢٣٤٥٦٧٨٩'.isdecimal()  # Arabic-Indic digits zero to nine
+      True
+      >>> 'alphabetic'.isdecimal()
+      False
 
 
 .. method:: str.isdigit()