Document what features are broken when db_key_base is lost

doc/raketasks/backup_restore.md

@@ -806,9 +806,22 @@ If you have failed to [back up the secrets file](#storing-configuration-files),
 then users with 2FA enabled will not be able to log into GitLab. In that case,
 you need to [disable 2FA for everyone](../security/two_factor_authentication.md#disabling-2fa-for-everyone).
 
-In the case of CI/CD, if your project has secure variables set, you might experience
-some weird behavior, like stuck jobs or 500 errors. In that case, you can try
-deleting the `ci_variables` table from the database.
+The secrets file is also responsible for storing the encryption key for several
+columns containing sensitive information. If the key is lost, GitLab will be
+unable to decrypt those columns. This will break a wide range of functionality,
+including (but not restricted to):
+
+* [CI/CD variables](../ci/variables/README.md)
+* [Kubernetes / GCP integration](../user/project/clusters/index.md)
+* [Custom Pages domains](../user/project/pages/getting_started_part_three.md)
+* [Project error tracking](../user/project/operations/error_tracking.md)
+* [Runner authentication](../ci/runners/README.md)
+* [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.
 
 CAUTION: **Warning:**
 Use the following commands at your own risk, and make sure you've taken a
@@ -828,9 +841,10 @@ backup beforehand.
     sudo -u git -H bundle exec rails dbconsole RAILS_ENV=production
     ```
 
-1.  Check the `ci_variables` table:
+1.  Check the `ci_group_variables` and `ci_variables` tables:
 
     ```sql
+    SELECT * FROM public."ci_group_variables";
     SELECT * FROM public."ci_variables";
     ```
 
@@ -839,6 +853,7 @@ backup beforehand.
 1.  Drop the table:
 
     ```sql
+    DELETE FROM ci_group_variables;
     DELETE FROM ci_variables;
     ```
 
@@ -848,5 +863,9 @@ backup beforehand.
 You should now be able to visit your project, and the jobs will start
 running again.
 
+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,
+and the lost data can be manually replaced.
+
 [reconfigure GitLab]: ../administration/restart_gitlab.md#omnibus-gitlab-reconfigure
 [restart GitLab]: ../administration/restart_gitlab.md#installations-from-source