doc: show how to use to git to submit smaller and faster PRs

doc/contribute/contributor_expectations.rst

@@ -11,14 +11,46 @@ benefits:
   to set aside a few minutes to review smaller changes several times than it is
   to allocate large blocks of time to review a large PR.
 
-- Less wasted work if reviewers or maintainers reject the direction of the
-  change.
+- Contentious and long discussions have a more restricted "splash damage"
+  because they cannot drown unrelated topics that have successfully escaped
+  in separate Pull Requests.
 
 - Easier to rebase and merge. Smaller PRs are less likely to conflict with other
   changes in the tree.
 
+- Mixing different authors in the same pull request adds :ref:`extra
+  complications <modifying_contributions>`.
+
+- When independent of each other, smaller PRs can progress *concurrently*.
+  Disagreements, nitpicks and other delays in one place do not hold back
+  everything else.
+
+- Even when smaller PRs are dependent of each other, you can start reviews
+  and getting code merged before the whole work is complete. This faster
+  approach is known as "Stacked Diffs".
+
+- Less wasted work if reviewers or maintainers reject the direction of the
+  change.
+
 - Easier to revert if the PR breaks functionality.
 
+- Better test coverage before merge because CI does not test intermediate
+  commits in a PR; only PRs as a whole.
+
+- Better test coverage after merge. Pull Request testing is limited
+  because it must provide feedback in a reasonable time. Typically: under
+  one or two hours. After merge, Zephyr-based projects, companies and
+  organizations test the main branch(es) again but for longer and with a
+  much higher level of stress. (Post-merge testing is often called "daily"
+  even when run several times a day.) Merging large changes in smaller
+  chunks provides a tighter and faster feedback loop and makes regressions
+  much more obvious. This is what "Continuous" means in "Continuous
+  Integration".
+
+The :ref:`Contribution workflow` section shows how to use git to submit
+several, smaller pull requests. This does not require creating and
+managing multiple git branches.
+
 .. note::
   This page does not apply to draft PRs which can have any size, any number of
   commits and any combination of smaller PRs for testing and preview purposes.

doc/contribute/guidelines.rst

@@ -838,8 +838,24 @@ workflow here:
 
      git push origin fix_comment_typo
 
+#. Avoid submitting a large number of commits in a single pull request,
+   see :ref:`contributor-expectations` why. It is tempting to submit
+   all at once and hope that everything will be merged faster but the
+   opposite effect is generally achieved. Submitting several, smaller
+   pull requests does *not* require creating and managing local git
+   branches::
+
+     git push origin big_work~15:refs/heads/big_work_part1
+
+   In this example, the 15 most recent commits in the ``big_work`` local
+   branch are *omitted* from the ``big_work_part1`` remote branch and pull
+   request. This "Stacked Diffs" approach lets you submit part 1 and get
+   reviews started long before the rest is ready.  When smaller parts are
+   independent of each other, rotating them with ``git rebase -i`` (see below)
+   lets you submit them concurrently which is even faster.
+
 #. In your web browser, go to your forked repo and click on the
-   ``Compare & pull request`` button for the branch you just worked on and
+   ``Compare & pull request`` button for the remote branch you just pushed and
    you want to open a pull request with.
 
 #. Review the pull request changes, and verify that you are opening a pull