Warn about restoring to the same version.

doc/raketasks/backup_restore.md

@@ -6,8 +6,8 @@
 
 A backup creates an archive file that contains the database, all repositories and all attachments.
 This archive will be saved in backup_path (see `config/gitlab.yml`).
-
 The filename will be `[TIMESTAMP]_gitlab_backup.tar`. This timestamp can be used to restore an specific backup.
+You can only restore a backup to exactly the same version of GitLab that you created it on, for example 7.2.1.
 
 ```
 # omnibus-gitlab
@@ -55,6 +55,8 @@ If you have a manual installation please consider backing up your gitlab.yml fil
 
 ## Restore a previously created backup
 
+You can only restore a backup to exactly the same version of GitLab that you created it on, for example 7.2.1.
+
 ```
 # omnibus-gitlab
 sudo gitlab-rake gitlab:backup:restore