Feature flag documentation for instance level vars

doc/api/instance_level_ci_variables.md

 # Instance-level CI/CD variables API
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14108) in GitLab 13.0
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14108) in GitLab 13.0
+> - It's deployed behind a feature flag, enabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-instance-level-cicd-variables-core-only). **(CORE ONLY)**
 
 ## List all instance variables
 
@@ -134,3 +138,22 @@ DELETE /admin/ci/variables/:key
 ```shell
 curl --request DELETE --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/admin/ci/variables/VARIABLE_1"
 ```
+
+### Enable or disable instance-level CI/CD variables **(CORE ONLY)**
+
+Instance-level CI/CD variables is under development but ready for production use.
+It is deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../administration/feature_flags.md)
+can opt to disable it for your instance.
+
+To disable it:
+
+```ruby
+Feature.disable(:ci_instance_level_variables)
+```
+
+To enable it:
+
+```ruby
+Feature.enable(:ci_instance_level_variables)
+```