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

Co-authored-by: Savannah Ostrowski <savannah@python.org>

Doc/library/argparse.rst

@@ -2032,6 +2032,9 @@ 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.
+
    .. deprecated-removed:: 3.11 3.14
       Calling :meth:`add_argument_group` on an argument group now raises an
       exception. This nesting was never supported, often failed to work