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

Documentation/process/submitting-patches.rst

@@ -790,10 +790,14 @@ Providing base tree information
 -------------------------------
 
 When other developers receive your patches and start the review process,
-it is often useful for them to know where in the tree history they
-should place your work. This is particularly useful for automated CI
-processes that attempt to run a series of tests in order to establish
-the quality of your submission before the maintainer starts the review.
+it is absolutely necessary for them to know what is the base
+commit/branch your work applies on, considering the sheer amount of
+maintainer trees present nowadays. Note again the **T:** entry in the
+MAINTAINERS file explained above.
+
+This is even more important for automated CI processes that attempt to
+run a series of tests in order to establish the quality of your
+submission before the maintainer starts the review.
 
 If you are using ``git format-patch`` to generate your patches, you can
 automatically include the base tree information in your submission by
@@ -836,6 +840,9 @@ letter or in the first patch of the series and it should be placed
 either below the ``---`` line or at the very bottom of all other
 content, right before your email signature.
 
+Make sure that base commit is in an official maintainer/mainline tree
+and not in some internal, accessible only to you tree - otherwise it
+would be worthless.
 
 References
 ----------