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

[Phil Sainty <psainty@orcon.net.nz>]

This feature dates to the following commit, which I believe was
first released in 24.1.

Because this seems such a useful feature, I'm inclined to add a
NEWS entry as well, with the note "This change was made in Emacs
24.1, but wasn't documented until now."


-Phil



commit b2e4481906ec08e2c6072b6fe14dd1efca4784b9
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Date:   Mon Nov 22 14:22:03 2010 -0500

     Cleanup diff-buffer-with-file.
     * lisp/vc/diff.el (diff-old-temp-file, diff-new-temp-file): Remove.
     (diff-sentinel): Get them as arguments instead.
     (diff-old-file, diff-new-file, diff-extra-args): Remove.
     (diff-file-local-copy, diff-better-file-name): New funs.
     (diff-no-select): Rename from diff-into-buffer.
     Support buffers additionally to files.  Move `buf' arg.  Don't 
display buf.
     Prefer closures to buffer-local variables.
     (diff): Adjust accordingly.
     (diff-buffer-with-file): Move from files.el.
     * lisp/files.el (diff-buffer-with-file): Move to vc/diff.el.
     (diff-buffer-internal): Remove.
     (diff-buffer-buffer): Remove.
     (save-some-buffers-action-alist): Use diff-no-select so as not to 
guess
     the buffer name used, and so as not to mess up windows and frames.