Add feature flag info to Admin mode in 13.10 docs

doc/user/admin_area/settings/sign_in_restrictions.md

@@ -25,6 +25,10 @@ You can restrict the password authentication for web interface and Git over HTTP
 
 ## Admin Mode
 
+> - [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2158) in GitLab 13.10.
+> - It's [deployed behind the feature flag](../../../user/feature_flags.md) `:user_mode_in_session`, disabled by default.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to enable it.
+
 When this feature is enabled, instance administrators are limited as regular users. During that period,
 they do not have access to all projects, groups, or the **Admin Area** menu.
 
@@ -47,7 +51,7 @@ OmniAuth providers and LDAP auth. The Admin Mode status is stored in the active
 session and remains active until it is explicitly disabled (it will be disabled
 automatically after a timeout otherwise).
 
-### Limitations
+### Limitations of Admin Mode
 
 The following access methods are **not** protected by Admin Mode:
 
@@ -61,7 +65,7 @@ authentication steps.
 We may address these limitations in the future. For more information see the following epic:
 [Admin mode for GitLab Administrators](https://gitlab.com/groups/gitlab-org/-/epics/2158).
 
-### Troubleshooting
+### Troubleshooting Admin Mode
 
 If necessary, you can disable **Admin Mode** as an administrator by using one of these two methods:
 
@@ -77,6 +81,25 @@ If necessary, you can disable **Admin Mode** as an administrator by using one of
   ::Gitlab::CurrentSettings.update_attributes!(admin_mode: false)
   ```
   
+## Enable or disable Admin Mode
+
+Admin Mode is under development and not ready for production use. It is
+deployed behind a feature flag that is **disabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can enable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:user_mode_in_session)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:user_mode_in_session)
+```
+
 ## Two-factor authentication
 
 When this feature is enabled, all users must use the [two-factor authentication](../../profile/account/two_factor_authentication.md).