Update pull request template (#2234)

.github/pull_request_template.md

@@ -1,9 +1,9 @@
 # Checklist for submitter
 
-If some of the following don't apply, please write a short explanation why.
+If some of the following don't apply, delete the relevant line.
 
-- [ ] Changes file added (if needed)
+- [ ] Changes file added (for user-visible changes)
 - [ ] Documented any API changes
 - [ ] Documented any permissions changes
-- [ ] Added tests for all functionality
-- [ ] Manual QA for all functionality
+- [ ] Added/updated tests
+- [ ] Manual QA for all new/changed functionality