Add instruction to specify group or project when deleting variables

doc/raketasks/backup_restore.md

@@ -1422,6 +1422,13 @@ after which users must reactivate 2FA.
    DELETE FROM ci_variables;
    ```
 
+1. If you know the specific group or project from which you wish to delete variables, you can include a `WHERE` statement to specify that in your `DELETE`:
+
+   ```sql
+   DELETE FROM ci_group_variables WHERE group_id = <GROUPID>;
+   DELETE FROM ci_variables WHERE project_id = <PROJECTID>;
+   ```
+
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