pythonGH-61082: Clarify nargs='*' positional default behavior (pythonGH-150989)

(cherry picked from commit bc37a22)

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

Author: savannahostrowski

Doc/library/argparse.rst

@@ -953,6 +953,10 @@ is used when no command-line argument was present::
    >>> parser.parse_args([])
    Namespace(foo=42)
 
+Because ``nargs='*'`` gathers any supplied values into a list, an absent
+positional argument yields an empty list (``[]``). Only a non-``None``
+*default* overrides this (so ``default=None`` still gives ``[]``).
+
 For required_ arguments, the ``default`` value is ignored. For example, this
 applies to positional arguments with nargs_ values other than ``?`` or ``*``,
 or optional arguments marked as ``required=True``.