gh-69223: Document that add_argument returns an Action object

Add documentation noting that ArgumentParser.add_argument() returns
an Action object, which can be used for further customization or
inspection of argument properties.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kovan

Doc/library/argparse.rst

@@ -706,6 +706,10 @@ The add_argument() method
 
    * deprecated_ - Whether or not use of the argument is deprecated.
 
+   The method returns an :class:`Action` object representing the argument.
+   This object can be used to further customize behavior or to inspect
+   argument properties.
+
 The following sections describe how each of these are used.