Document Test Failure History feature

doc/ci/unit_test_reports.md

@@ -65,6 +65,39 @@ execution time and the error output.
 
 ![Test Reports Widget](img/junit_test_report.png)
 
+### Number of recent failures
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/241759) in GitLab 13.7.
+> - It's [deployed behind a feature flag](../user/feature_flags.md), disabled by default.
+> - It's disabled on GitLab.com.
+> - It's not recommended for production use.
+> - To use it in GitLab self-managed instances, ask a GitLab administrator to [enable it](#enable-or-disable-the-number-of-recent-failures). **(CORE ONLY)**
+
+WARNING:
+This feature might not be available to you. Check the **version history** note above for details.
+
+If a test failed in the project's default branch in the last 14 days, a message like
+`Failed {n} time(s) in {default_branch} in the last 14 days` is displayed for that test.
+
+#### Enable or disable the number of recent failures **(CORE ONLY)**
+
+Displaying the number of failures in the last 14 days 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(:test_failure_history)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:test_failure_history)
+```
+
 ## How to set it up
 
 To enable the Unit test reports in merge requests, you need to add