gh-106318: Add example for str.swapcase() method (#144575)

Doc/library/stdtypes.rst

@@ -2775,8 +2775,22 @@ expression support in the :mod:`re` module).
 .. method:: str.swapcase()
 
    Return a copy of the string with uppercase characters converted to lowercase and
-   vice versa. Note that it is not necessarily true that
-   ``s.swapcase().swapcase() == s``.
+   vice versa. For example:
+
+   .. doctest::
+
+      >>> 'Hello World'.swapcase()
+      'hELLO wORLD'
+
+   Note that it is not necessarily true that ``s.swapcase().swapcase() == s``.
+   For example:
+
+   .. doctest::
+
+      >>> 'straße'.swapcase().swapcase()
+      'strasse'
+
+   See also :meth:`str.lower` and :meth:`str.upper`.
 
 
 .. method:: str.title()