gh-118359: Improve docs for Bdb.user_call (#118368)

The `argument_list` parameter of bdb.Bdb.user_call has been useless for 25 years. It is retained for backwards compatibility, but it will always be None.

Author: gaogaotiantian

Doc/library/bdb.rst

@@ -240,6 +240,9 @@ The :mod:`bdb` module also defines two classes:
       Called from :meth:`dispatch_call` if a break might stop inside the
       called function.
 
+      *argument_list* is not used anymore and will always be ``None``.
+      The argument is kept for backwards compatibility.
+
    .. method:: user_line(frame)
 
       Called from :meth:`dispatch_line` when either :meth:`stop_here` or