UI: Inform user what meaning hourglass on reviews has (#12713)

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

options/locale/locale_en-US.ini

@@ -1027,6 +1027,7 @@ issues.poster = Poster
 issues.collaborator = Collaborator
 issues.owner = Owner
 issues.re_request_review=Re-request review
+issues.is_stale = There have been changes to this PR since this review
 issues.remove_request_review=Remove review request
 issues.remove_request_review_block=Can't remove review request
 issues.sign_in_require_desc = <a href="%s">Sign in</a> to join this conversation.