Add a troubleshooting section for backup restore docs, explain about warnings...

Add a troubleshooting section for backup restore docs, explain about warnings when using omnibus packages.

doc/raketasks/backup_restore.md

@@ -300,6 +300,25 @@ Example: LVM snapshots + rsync
 If you are running GitLab on a virtualized server you can possibly also create VM snapshots of the entire GitLab server.
 It is not uncommon however for a VM snapshot to require you to power down the server, so this approach is probably of limited practical use.
 
-### Note
+## Troubleshooting
+
+### Restoring database backup using omnibus packages outputs warnings
+If you are using backup restore procedures you might encounter the following warnings:
+
+```
+psql:/var/opt/gitlab/backups/db/database.sql:22: ERROR:  must be owner of extension plpgsql
+psql:/var/opt/gitlab/backups/db/database.sql:2931: WARNING:  no privileges could be revoked for "public" (two occurences)
+psql:/var/opt/gitlab/backups/db/database.sql:2933: WARNING:  no privileges were granted for "public" (two occurences)
+
+```
+
+Be advised that, backup is successfully restored in spite of these warnings.
+
+The rake task runs this as the `gitlab` user which does not have the superuser access to the database. When restore is initiated it will also run as `gitlab` user but it will also try to alter the objects it does not have access to.
+Those objects have no influence on the database backup/restore but they give this annoying warning.
+
+For more information see similar questions on postgresql issue tracker[here](http://www.postgresql.org/message-id/201110220712.30886.adrian.klaver@gmail.com) and [here](http://www.postgresql.org/message-id/2039.1177339749@sss.pgh.pa.us) as well as [stack overflow](http://stackoverflow.com/questions/4368789/error-must-be-owner-of-language-plpgsql).
+
+## Note
 This documentation is for GitLab CE.
 We backup GitLab.com and make sure your data is secure, but you can't use these methods to export / backup your data yourself from GitLab.com.