document two diagnostics commands i found out about while filing a bug

howto/gitlab.md

@@ -396,6 +396,21 @@ this will be implemented, if ever.
    * [GitLab overview](https://grafana.torproject.org/d/QrDJktiMz/gitlab-omnibus)
    * [Gitaly](https://grafana.torproject.org/d/x6Z50y-iz/gitlab-gitaly)
 
+TODO: document how to handle common problems in GitLab
+
+### Troubleshooting
+
+Upstream recommends running this command to self-test a GitLab
+instance:
+
+    sudo gitlab-rake gitlab:check SANITIZE=true
+
+This command also shows general info about the GitLab instance:
+
+    sudo gitlab-rake gitlab:check SANITIZE=true
+
+it is especially useful to find on-disk files and package versions.
+
 ## Disaster recovery
 
 In case the entire GitLab machine is destroyed, a new server should be