Updates docs and adds changelog

changelogs/unreleased/lm-enable-one-d-matrix-by-default.yml

+---
+title: Enable one_dimensional_matrix feature flag by default
+merge_request: 45086
+author:
+type: added

doc/ci/yaml/README.md

@@ -3587,7 +3587,8 @@ There can be from 2 to 50 jobs.
 [In GitLab 13.5](https://gitlab.com/gitlab-org/gitlab/-/issues/26362) and later,
 you can have one-dimensional matrices with a single job.
 The ability to have one-dimensional matrices is [deployed behind a feature flag](../../user/feature_flags.md),
-enabled by default. It's enabled on GitLab.com.  **(CORE ONLY)**
+enabled by default. It's enabled on GitLab.com. For self-managed GitLab instances,
+administrators can opt to disable it by [disabling the `one_dimensional_matrix:` feature flag](../../administration/feature_flags.md). **(CORE ONLY)**
 
 Every job gets the same `CI_NODE_TOTAL` [environment variable](../variables/README.md#predefined-environment-variables) value, and a unique `CI_NODE_INDEX` value.