gh-69223: Document that add_argument returns an Action object

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kovan

Doc/library/argparse.rst

@@ -698,6 +698,8 @@ The add_argument() method
 
    * deprecated_ - Whether or not use of the argument is deprecated.
 
+   The method returns an :class:`Action` object representing the argument.
+
 The following sections describe how each of these are used.