Merge branch 'add_to_backup_docs' into 'master'

Explain warnings when using omnibus packages with backup restore

Instead of adding this to the omnibus documentation as proposed [here](https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/392), moving to one central place as part of efforts to have central location for the docs.

See merge request !903

doc/raketasks/backup_restore.md

@@ -19,7 +19,7 @@ sudo gitlab-rake gitlab:backup:create
 sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
 ```
 
-Also you can choose what should be backed up by adding environment variable SKIP. Available options: db, 
+Also you can choose what should be backed up by adding environment variable SKIP. Available options: db,
 uploads (attachments), repositories. Use a comma to specify several options at the same time.
 
 ```
@@ -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
-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.
\ No newline at end of file
+## 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.