github: give greater instructions for commit messages (#7904)

This brings the commit message section of the PULL_REQUEST_TEMPLATE.md
into line with the text used in the beats repository.

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,10 +6,25 @@ Please label this PR with one of the following labels, depending on the scope of
 - Deprecation
 -->
 
-## What does this PR do?
+## Proposed commit message
 
 <!-- Mandatory
 Explain here the changes you made on the PR.
+
+Please explain:
+
+- 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
@@ -24,7 +39,7 @@ Explain here the changes you made on the PR.
 <!-- Recommended
 Add a checklist of things that are required to be reviewed in order to have the PR approved
 -->
-- [ ]
+- [ ] 
 
 ## How to test this PR locally
 
@@ -42,7 +57,7 @@ Link related issues below. Insert the issue link or reference after the word "Cl
 - Requires #123
 - Supersedes #123
 -->
--
+- 
 
 ## Screenshots