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>

Author: vegard

Documentation/doc-guide/contributing.rst

@@ -138,6 +138,10 @@ times, but it's highly important.  If we can actually eliminate warnings
 from the documentation build, then we can start expecting developers to
 avoid adding new ones.
 
+In addition to warnings from the regular documentation build, you can also
+run ``make refcheckdocs`` to find references to nonexistent documentation
+files.
+
 Languishing kerneldoc comments
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~