bug#38575: 27.0.50; Document that `diff' arguments OLD and NEW can be buffers

[Eli Zaretskii]

> Cc: 38575@debbugs.gnu.org
> From: Phil Sainty <psainty@orcon.net.nz>
> Date: Thu, 12 Dec 2019 23:15:37 +1300
> 
> The user might otherwise wonder whether Emacs is actually using the
> external diff program in this case (vs using some internal functions
> to process the buffer text directly to produce a diff).

They have the sources to solve this confusion.

> I'm ok with omitting it if you're not convinced.

I'd prefer to omit that, because we may one day find a way of running
programs with input redirected from a buffer, or something.  Not a
strong opinion, though.

Thanks.