[3.12] gh-118359: Improve docs for Bdb.user_call (GH-118368) (#118410)

gh-118359: Improve docs for Bdb.user_call (GH-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.
(cherry picked from commit 8e4fb5d)

Co-authored-by: Tian Gao <[email protected]>

Author: miss-islington

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