SubmittingPatches: encourage trailer use for substantial help

Trailers beyond the mandatory s-o-b are regularly used based on my
last two years of reading the mailing list. Moreover, reviewers might
encourage it.[1]

This is also in line with the project crediting both commit authors and
people mentioned in trailers each release; “Nobody is THE one making
contribution”.[2]

Adding trailers is already encouraged, but in the section `send-patches`.
Let’s replace “If you like” with outright encouragement in this section
so that all trailer discussion (except s-o-b; see `sign-off` section) is
contained in this section; a link to from `send-patches` makes this
information equally visible.

Now we need to make a heading for `commit-trailers` in order for the
HTML output to make sense.

At the same time, it is important to temper this recommendation to a
significant enough contribution; in my experience beginners can be eager
to add a trailer for everyone who replies with an action point that is
followed up on.

Let’s also spell out that these trailers should follow the Git author/
committer format. One might naturally just write the name, but in that
case it will not be picked up by:

    git shortlog --group=trailer:<key>

and normalization via `.mailmap` will not work.

Also introduce the list of common trailers as such. Granted, this is
already implied by the later paragraph about “create your own trailer”,
so this just frontloads this information.

† 1: https://lore.kernel.org/git/CAP8UFD0POvYDgGtEx8GBhvKkd8XzzWQsy8XxAKL9M3+uz3ka+w@mail.gmail.com/#:~:text=for%20at%20least
† 2: https://lore.kernel.org/git/xmqqzh248sy0.fsf@gitster.c.googlers.com/

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -444,7 +444,15 @@ The goal of this policy is to allow us to have sufficient information to contact
 you if questions arise about your contribution.
 
 [[commit-trailers]]
-If you like, you can put extra trailers at the end:
+=== Commit trailers
+It is polite to credit people who have helped with your work to a
+substantial enough degree. This project uses commit trailers for that,
+where the credited person is written out like a Git author, i.e. with
+both their name and their email address. Note that the threshold to
+credit someone is a judgement call, and crediting someone for simple
+review work is certainly not necessary.
+
+These are the common trailers in use:
 
 . `Reported-by:` is used to credit someone who found the bug that
   the patch attempts to fix.
@@ -562,8 +570,8 @@ when the maintainer did not heavily participate in the discussion and
 instead left the review to trusted others.
 
 Do not forget to add trailers such as `Acked-by:`, `Reviewed-by:` and
-`Tested-by:` lines as necessary to credit people who helped your
-patch, and "cc:" them when sending such a final version for inclusion.
+`Tested-by:` (see <<commit-trailers,Commit trailers>>), and "cc:" them
+when sending such a final version for inclusion.
 
 ==== `format-patch` and `send-email`