docs: submitting-patches: improve the base commit explanation

After receiving a second patchset this week without knowing which tree
it applies on and trying to apply it on the obvious ones and failing,
make sure the base tree information which needs to be supplied in the
0th message of the patchset is spelled out more explicitly.

Also, make the formulations stronger as this really is a requirement and
not only a useful thing anymore.

Signed-off-by: "Borislav Petkov (AMD)" <bp@alien8.de>
change-id: <unique-series-id>
base-commit: <commit-id-or-tag>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231115170330.16626-1-bp@alien8.de