Added a feature flag mention to design docs

doc/user/project/issues/design_management.md

@@ -71,12 +71,40 @@ and connect to GitLab through a personal access token. The details are explained
 
 ## The Design Management section
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/223193) in GitLab 13.2, Designs are displayed directly on the issue description rather than on a separate tab.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/223193) in GitLab 13.2, Designs are displayed directly on the issue description rather than on a separate tab.
+> - The new display is deployed behind a feature flag, enabled by default.
+> - It's enabled on GitLab.com.
+> - It cannot be enabled or disabled per-project.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-displaying-designs-on-the-issue-description-core-only). If disabled, it will move Designs back to the **Designs** tab.
 
 You can find to the **Design Management** section in the issue description:
 
 ![Designs section](img/design_management_v13_2.png)
 
+### Enable or disable displaying Designs on the issue description **(CORE ONLY)**
+
+Displaying Designs on the issue description 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(:design_management_moved)
+```
+
+To enable it:
+
+```ruby
+Feature.enable(:design_management_moved)
+```
+
+By disabling this feature, designs will be displayed on the **Designs** tab
+instead of directly on the issue description.
+
 ## Adding designs
 
 To upload Design images, drag files from your computer and drop them in the Design Management section,