Merge branch 'update-matrix-builds-docs' into 'master'

Update docs for matrix builds

See merge request gitlab-org/gitlab!37698

doc/ci/yaml/README.md

@@ -3438,7 +3438,11 @@ job split into three separate jobs.
 
 #### Parallel `matrix` jobs
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15356) in GitLab 13.2.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15356) in GitLab 13.3.
+> - It's deployed behind a feature flag, disabled by default.
+> - It's enabled on GitLab.com.
+> - It can't be enabled or disabled per-project.
+> - It's recommended for production use.
 
 `matrix:` allows you to configure different variables for jobs that are running in parallel.
 There can be from 2 to 50 jobs.