bug#54296: Add buffer-matching functionality

[Eli Zaretskii]

> From: Philip Kaludercic <philipk@posteo.net>
> Cc: larsi@gnus.org,  54296@debbugs.gnu.org
> Date: Sun, 13 Mar 2022 20:40:49 +0000
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Philip Kaludercic <philipk@posteo.net>
> >> Cc: larsi@gnus.org,  54296@debbugs.gnu.org
> >> Date: Fri, 11 Mar 2022 16:21:06 +0000
> >> 
> >> > I think we want in general avoid comparison with major-mode, and
> >> > prefer derived-mode instead, and if so, IMO we had better did as we
> >> > say and not exposed comparison to major mode unless we absolutely
> >> > must.
> >> 
> >> Would it be enough to clarify this point in the documentation string?
> >
> > What would you like to clarify?
> 
> To clarify that the usage of (major-mode . foo-mode) might not be what
> the user intends, and that in most cases derived-mode is preferable.

I suggested to "clarify" that by not providing the 'major-mode'
predicate at all.  I still don't think I understand why it is so
important that we should provide a special case for it.