bug#39149: 27.0.50; describe-buffer-bindings is calling :filter function in wrong buffer

[Eli Zaretskii]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Sat, 14 Nov 2020 16:24:07 -0800
> Cc: 39149@debbugs.gnu.org
> 
> a) Add a new argument to describe-map with the original buffer.  Ensure
>    that the :filter function is run there.
> 
> b) Change describe-map to return its output as a string.

Unless I'm missing something, b) would be a backward-incompatible
change, so I don't think we should consider it.

If adding an argument is deemed not a good idea, how about binding a
special variable to convey that information to
describe-buffer-bindings?