test(labels): document sibling label fixture

Author: kolaente

pkg/db/fixtures/labels.yml

@@ -64,6 +64,7 @@
   created_by_id: 21
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+# Must remain unattached so sibling-access tests exercise identity access, not task access.
 - id: 12
   title: 'Label #12 - created by bot 25, sibling of bot 23'
   created_by_id: 25