bug#54296: Add buffer-matching functionality

[Eli Zaretskii]

> Date: Fri, 17 Jun 2022 04:21:04 +0300
> Cc: 54296@debbugs.gnu.org, philipk@posteo.net, larsi@gnus.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> 
> > In any case, if we will keep both major-mode and derived-mode
> > conditions in subr.el, let's at least document that derived-mode
> > should be preferred where it can do the job, and major-mode used only
> > where derived-mode will not DTRT.
> 
> Sure. Here you go.

Fine with me, with one nit:

> +Satisfied if the buffer's major mode is eq to @var{expr}.  Prefer

Using "eq" as a word in the manual is not a good idea.  We use "is
equal" elsewhere in the manual.

Thanks.