bpo-38531: document extend action's added version (pythonGH-16865) (pythonGH-16868)

(cherry picked from commit 7414207)

Co-authored-by: Batuhan Taşkaya <47358913+isidentical@users.noreply.github.com>

Author: 2 people

Doc/library/argparse.rst

@@ -810,6 +810,8 @@ how the command-line arguments should be handled. The supplied actions are:
     >>> parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"])
     Namespace(foo=['f1', 'f2', 'f3', 'f4'])
 
+  .. versionadded:: 3.8
+
 You may also specify an arbitrary action by passing an Action subclass or
 other object that implements the same interface.  The recommended way to do
 this is to extend :class:`Action`, overriding the ``__call__`` method