Add some examples of when to remove the Sheriff-Chromium label.

Change-Id: I2ae2cb55b9fd0f48e7af1a5f8c77983840a42d0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2256919
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781086}

docs/sheriff.md

@@ -302,7 +302,10 @@ If yes:
 * Find the relevant Slack thread or start a new one
 * Investigate the bug and see what needs to happen - ping the owner, see if the
   priority & assignment are right, or try fixing it yourself. Check if it still
-  needs to be in the queue!
+  needs to be in the queue! If it doesn't need to be in the queue, remove the
+  `Sheriff-Chromium` label. Do this when
+  * the test has been disabled or marked flaky
+  * the problem is no longer impacting any of the builders
 
 ### 5. All's well?