github: unify PR description and commit message (#36205)

To improve the quality of commit messages in the revision history,
indicate that the commit message will be part of the PR review process
and integrate the WHAT and WHY questions in the change explanation.

Remove the checklist item for PR title and description as it becomes
redundant with this new text.

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,16 +8,25 @@ Please label this PR with one of the following labels, depending on the scope of
 - Docs
 -->
 
-## What does this PR do?
+## Proposed commit message
 
 <!-- Mandatory
-Explain here the changes you made on the PR. Please explain the WHAT: patterns used, algorithms implemented, design architecture, message processing, etc.
--->
+Explain here the changes you made on the PR.
 
-## Why is it important?
+Please explain:
 
-<!-- Mandatory
-Explain here the WHY, or the rationale/motivation for the changes.
+- WHAT: patterns used, algorithms implemented, design architecture, message processing, etc.
+- WHY:  the rationale/motivation for the changes
+
+This text will be pasted into the squash dialog when the change is committed and will be
+a long term historical record of the change to help future contributors understand the
+change, please help them by making it clear and comprehensive, they may be you.
+
+If the commit title is adequate to describe both of these things, The text here may be omitted
+or replaced with "See title". The title of the PR will be used as the commit message title when
+the merge is made and the "See title" marker will be removed if present.
+
+The text here and the PR title will be subject to the PR review process.
 -->
 
 ## Checklist
@@ -34,7 +43,6 @@ List here all the items you have verified BEFORE sending this PR. Please DO NOT
 - [ ] I have made corresponding change to the default configuration files
 - [ ] I have added tests that prove my fix is effective or that my feature works
 - [ ] I have added an entry in `CHANGELOG.next.asciidoc` or `CHANGELOG-developer.next.asciidoc`.
-- [ ] I have made my commit title and message explanatory about the purpose and the reason of the change
 
 ## Author's Checklist