docs: process: Add base-commit trailer usage

One of the recurring complaints from both maintainers and CI system
operators is that performing git-am on received patches is difficult
without knowing the parent object in the git history on which the
patches are based. Without this information, there is a high likelihood
that git-am will fail due to conflicts, which is particularly
frustrating to CI operators.

Git versions starting with v2.9.0 are able to automatically include
base-commit information using the --base flag of git-format-patch.
Document this usage in process/submitting-patches, and add the rationale
for its inclusion, plus instructions for those not using git on where
the "base-commit:" trailer should go.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>