Documentation: submit-checklist: add clean builds for new Documentation

Add to Documentation/process/submit-checklist.rst that patch
submitters should run "make htmldocs" and verify that any
Documentation/ changes (patches) are clean (no new warnings/errors).
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/cf5bbdf5-03ff-0606-a6d4-ca196d90aee9@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/submit-checklist.rst

@@ -24,6 +24,10 @@ and elsewhere regarding submitting Linux kernel patches.
 
   c) Builds successfully when using ``O=builddir``
 
+  d) Any Documentation/ changes build successfully without new warnings/errors.
+     Use ``make htmldocs`` or ``make pdfdocs`` to check the build and
+     fix any issues.
+
 3) Builds on multiple CPU architectures by using local cross-compile tools
    or some other build farm.