update argparse's doc of append action. (#92344)

Author: shihai1991

Doc/library/argparse.rst

@@ -846,8 +846,10 @@ how the command-line arguments should be handled. The supplied actions are:
     Namespace(foo=True, bar=False, baz=True)
 
 * ``'append'`` - This stores a list, and appends each argument value to the
-  list.  This is useful to allow an option to be specified multiple times.
-  Example usage::
+  list. It is useful to allow an option to be specified multiple times.
+  If the default value is non-empty, the default elements will be present
+  in the parsed value for the option, with any values from the
+  command line appended after those default values. Example usage::
 
     >>> parser = argparse.ArgumentParser()
     >>> parser.add_argument('--foo', action='append')