docs: mention that the changelog block in PR is special

Author: henryiii

.github/pull_request_template.md

@@ -5,7 +5,8 @@
 
 ## Suggested changelog entry:
 
-<!-- fill in the below block with the expected RestructuredText entry (delete if no entry needed) -->
+<!-- Fill in the below block with the expected RestructuredText entry. Delete if no entry needed;
+     but do not delete header or rst block if an entry is needed! Will be collected via a script. -->
 
 ```rst