Update codequality documentation

doc/user/project/merge_requests/code_quality.md

@@ -49,6 +49,26 @@ For one customer, the auditor found that having Code Quality, SAST, and Containe
 
 See also the Code Climate list of [Supported Languages for Maintainability](https://docs.codeclimate.com/docs/supported-languages-for-maintainability).
 
+## Code Quality in diff view **(ULTIMATE)**
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/267612) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 13.11.
+> - [Deployed behind a feature flag](../../../user/feature_flags.md), disabled by default.
+
+Changes to files in merge requests can cause Code Quality to fall if merged. In these cases,
+an indicator is displayed (**{information-o}** **Code Quality**) on the file in the merge request's diff view. For example:
+
+![Code Quality MR diff report](img/code_quality_mr_diff_report_v13_11.png)
+
+To enable this feature, a GitLab administrator can run the following in a
+[Rails console](../../../administration/operations/rails_console.md):
+
+```ruby
+# For the instance
+Feature.enable(:codequality_mr_diff)
+# For a single project
+Feature.enable(:codequality_mr_diff, Project.find(<project id>))
+```
+
 ## Use cases
 
 For instance, consider the following workflow: