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: Sat, 14 Dec 2019 21:22:13 +1300
> 
> I ended up adding a `diff-buffers' command to provide better user
> support for this feature.
> 
> Patch attached for review.

LGTM, thanks.