docs: doc-guide: mention 'make refcheckdocs'

Add this to the section on fixing warnings.

Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Vegard Nossum <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

Documentation/doc-guide/contributing.rst

···
 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
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