Docs: SubmittingPatches: mention using pull requests as a cover letter

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/SubmittingPatches

@@ -725,7 +725,9 @@ maintainer pull them directly into the subsystem repository with a
 "git pull" operation.  Note, however, that pulling patches from a developer
 requires a higher degree of trust than taking patches from a mailing list.
 As a result, many subsystem maintainers are reluctant to take pull
-requests, especially from new, unknown developers.
+requests, especially from new, unknown developers.  If in doubt you can use
+the pull request as the cover letter for a normal posting of the patch
+series, giving the maintainer the option of using either.
 
 A pull request should have [GIT] or [PULL] in the subject line.  The
 request itself should include the repository name and the branch of