Clarify that docs-only changes don't need release notes

Author: hashhar

.github/pull_request_template.md

@@ -12,7 +12,7 @@
 <!-- Mark the appropriate option with an (x). Propose a release note if you can. -->
 ## Release notes
 
-( ) This is not user-visible and no release notes are required.
+( ) This is not user-visible or docs only and no release notes are required.
 ( ) Release notes are required, please propose a release note for me.
 ( ) Release notes are required, with the following suggested text: