gh-69223: Document that add_argument() returns an Action object (GH-145538)

(cherry picked from commit b28e5f5)

Co-authored-by: Andrew Barnes <bortstheboat@gmail.com>

Author: Bortlesboat

Doc/library/argparse.rst

@@ -613,6 +613,8 @@ 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.