pythongh-118508: Clarify which characters are matched by \s (python…

…#119155)

Clarify re syntax

Doc/library/re.rst

@@ -600,10 +600,9 @@ character ``'$'``.
 
 ``\s``
    For Unicode (str) patterns:
-      Matches Unicode whitespace characters (which includes
-      ``[ \t\n\r\f\v]``, and also many other characters, for example the
-      non-breaking spaces mandated by typography rules in many
-      languages).
+      Matches Unicode whitespace characters (as defined by :py:meth:`str.isspace`).
+      This includes ``[ \t\n\r\f\v]``, and also many other characters, for example the
+      non-breaking spaces mandated by typography rules in many languages.
 
       Matches ``[ \t\n\r\f\v]`` if the :py:const:`~re.ASCII` flag is used.