pythongh-69223: Document that add_argument() returns an Action object (python#145538)

Author: Bortlesboat

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.