Add doc for MR approvals scoping

This feature allows administrator to scope MR approvals settings to
compliance-labeled projects.

doc/user/admin_area/merge_requests_approvals.md

@@ -34,3 +34,37 @@ Merge request approval rules that can be set at an instance level are:
 - **Prevent users from modifying merge request approvers list**. Prevents project
   maintainers from allowing users to modify the approvers list in project settings
   or in individual merge requests.
+
+## Scope rules to compliance-labeled projects
+
+> Introduced in [GitLab Premium](https://gitlab.com/groups/gitlab-org/-/epics/3432) 13.1.
+
+Merge request approval rules can be further scoped to specific compliance frameworks.
+
+When the compliance framework label is selected and the project is assigned the compliance
+label, the instance-level MR approval settings will take effect and
+[project-level settings](../project/merge_requests/merge_request_approvals.md#adding--editing-a-default-approval-rule)
+is locked for modification.
+
+When the compliance framework label is not selected or the project is not assigned the
+compliance label, the project-level MR approval settings will take effect and the users with
+Maintainer role and above can modify these.
+
+| Instance-level | Project-level |
+| -------------- | ------------- |
+| ![Scope MR approval settings to compliance frameworks](img/scope_mr_approval_settings_v13_1.png) | ![MR approval settings on compliance projects](img/mr_approval_settings_compliance_project_v13_1.png) |
+
+### Enabling the feature
+
+This feature comes with two feature flags which are disabled by default.
+
+- The configuration in Admin area is controlled via `admin_merge_request_approval_settings`
+- The application of these rules is controlled via `project_merge_request_approval_settings`
+
+These feature flags can be managed by feature flag [API endpoint](../../api/features.md#set-or-create-a-feature) or
+by [GitLab administrators with access to the GitLab Rails console](../../administration/feature_flags.md) with the following commands:
+
+```ruby
+Feature.enable(:admin_merge_request_approval_settings)
+Feature.enable(:project_merge_request_approval_settings)
+```