[3.13] GH-108009: Add clarification of parser and argument defaults in argparse docs (GH-124154) (#141085)

GH-108009: Add clarification of parser and argument defaults in argparse docs (GH-124154)
(cherry picked from commit 95f6e1275b)

Co-authored-by: Savannah Ostrowski <savannah@python.org>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>

Doc/library/argparse.rst

@@ -1965,7 +1965,9 @@ Parser defaults
      >>> parser.parse_args(['736'])
      Namespace(bar=42, baz='badger', foo=736)
 
-   Note that parser-level defaults always override argument-level defaults::
+   Note that defaults can be set at both the parser level using :meth:`set_defaults`
+   and at the argument level using :meth:`add_argument`. If both are called for the
+   same argument, the last default set for an argument is used::
 
      >>> parser = argparse.ArgumentParser()
      >>> parser.add_argument('--foo', default='bar')