pythongh-72406: Document argument ordering in argparse help output (python#148534)

Co-authored-by: Savannah Ostrowski <savannah@python.org>
(cherry picked from commit 4286227)

Author: santihdzs

Doc/library/argparse.rst

@@ -1888,6 +1888,9 @@ Argument groups
    Note that any arguments not in your user-defined groups will end up back
    in the usual "positional arguments" and "optional arguments" sections.
 
+   Within each argument group, arguments are displayed in help output in the
+   order in which they are added.
+
    .. versionchanged:: 3.11
     Calling :meth:`add_argument_group` on an argument group is deprecated.
     This feature was never supported and does not always work correctly.