docs: Add a warning to applying-patches.rst

This is ancient stuff and we don't do things this way anymore.  In the
absence of simply deleting the document, at least add a warning to it.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed