Document Gitlab disallowing merge when there are no CI jobs

.gitlab-ci.yml

@@ -155,4 +155,5 @@ terminate:
     - python3 -c "import cosmos; cosmos.terminate_droplets_by_name(\"^ci-\", 2)"
   # This job has no `only` (anymore), so will always run. This works around a
   # Gitlab bug: if no job runs at all due to `only`, Gitlab gets confused and
-  # doesn't allow you to merge the MR.
+  # doesn't allow you to merge the MR:
+  # https://docs.gitlab.com/ee/user/project/merge_requests/merge_when_pipeline_succeeds.html#limitations