Add guide on how to reset runners token

doc/raketasks/backup_restore.md

@@ -848,15 +848,24 @@ including (but not restricted to):
 * [Project mirroring](../workflow/repository_mirroring.md)
 * [Web hooks](../user/project/integrations/webhooks.md)
 
-In the case of CI/CD, variables, you might experience some weird behavior, like
-stuck jobs or 500 errors. In that case, you can try removing  contents of the
-`ci_group_variables` and `ci_project_variables` tables from the database.
+In cases like CI/CD variables and Runner authentication, you might
+experience some unexpected behavior such as:
+
+- Stuck jobs.
+- 500 errors.
+
+In this case, you are required to reset all the tokens for CI/CD variables
+and Runner Authentication, which is described in more detail below. After
+resetting the tokens, you should be able to visit your project and the jobs
+will have started running again.
 
 CAUTION: **Warning:**
 Use the following commands at your own risk, and make sure you've taken a
 backup beforehand.
 
-1.  Enter the Rails console:
+#### Reset CI/CD variables
+
+1.  Enter the DB console:
 
     For Omnibus GitLab packages:
 
@@ -889,8 +898,39 @@ backup beforehand.
 1. You may need to reconfigure or restart GitLab for the changes to take
    effect.
 
-You should now be able to visit your project, and the jobs will start
-running again.
+
+#### Reset Runner registration tokens
+
+1.  Enter the DB console:
+
+    For Omnibus GitLab packages:
+
+    ```sh
+    sudo gitlab-rails dbconsole
+    ```
+
+    For installations from source:
+
+    ```sh
+    sudo -u git -H bundle exec rails dbconsole RAILS_ENV=production
+    ```
+
+1. Clear all the tokens for projects, groups, and the whole instance:
+
+CAUTION: **Caution:**
+The last UPDATE operation will stop the runners being able to pick up
+new jobs. You must register new runners.
+
+    ```sql
+    -- Clear project tokens
+    UPDATE projects SET runners_token = null, runners_token_encrypted = null;
+    -- Clear group tokens
+    UPDATE namespaces SET runners_token = null, runners_token_encrypted = null;
+    -- Clear instance tokens
+    UPDATE application_settings SET runners_registration_token_encrypted = null;
+    -- Clear runner tokens
+    UPDATE ci_runners SET token = null, token_encrypted = null;
+    ```
 
 A similar strategy can be employed for the remaining features - by removing the
 data that cannot be decrypted, GitLab can be brought back into working order,