docs: doc-guide: mention 'make refcheckdocs'

Add this to the section on fixing warnings.

Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20231022184910.919201-1-vegard.nossum@oracle.com>