[3.15] gh-69619: Clarify whitespace definition in str.strip docs (#150049)

(cherry picked from commit 17eb17d)

Co-authored-by: Daniil <d.mayorov@innopolis.university>

Author: miss-islington

Doc/library/stdtypes.rst

@@ -2747,6 +2747,8 @@ expression support in the :mod:`re` module).
    The *chars* argument is not a prefix or suffix; rather, all combinations of its
    values are stripped.
 
+   Whitespace characters are defined by :meth:`str.isspace`.
+
    For example:
 
    .. doctest::