chore(.github/PULL_REQUEST_TEMPLATE.md): clarify instructions (#6222)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,7 +1,14 @@
 
+
 ---
-<!--
-put comments you want to keep out of the PR commit here.
+<!-- The text above the `---` will become the commit message when your
+PR is merged. Please leave a blank newline before the `---`, otherwise
+GitHub will format the text above it as a title.
+
+Any other comments you want to keep out of the PR commit should go
+below the `---`, and placed outside this HTML comment, or else they
+will be invisible to reviewers.
+
 If this PR depends on other PRs, please list them below this comment,
 using the following format:
 - [ ] depends on: #abc [optional extra text]