Document "default_branch_name" application setting

doc/api/settings.md

@@ -245,6 +245,7 @@ listed in the descriptions of the relevant settings.
 | `container_registry_token_expire_delay`  | integer          | no                                   | Container Registry token duration in minutes. |
 | `deactivate_dormant_users`               | boolean          | no                                   | Enable [automatic deactivation of dormant users](../user/admin_area/moderate_users.md#automatically-deactivate-dormant-users). |
 | `default_artifacts_expire_in`            | string           | no                                   | Set the default expiration time for each job's artifacts. |
+| `default_branch_name`                    | string           | no                                   | [Instance-level custom initial branch name](../user/project/repository/branches/default.md#instance-level-custom-initial-branch-name) ([introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/225258) in GitLab 13.2). |
 | `default_branch_protection`              | integer          | no                                   | Determine if developers can push to the default branch. Can take: `0` _(not protected, both developers and maintainers can push new commits, force push, or delete the branch)_, `1` _(partially protected, developers and maintainers can push new commits, but cannot force push, or delete, the branch)_ or `2` _(fully protected, developers cannot push new commits, but maintainers can; no-one can force push or delete the branch)_ as a parameter. Default is `2`. |
 | `default_ci_config_path`                  | string           | no                                   | Default CI/CD configuration file and path for new projects (`.gitlab-ci.yml` if not set). |
 | `default_group_visibility`               | string           | no                                   | What visibility level new groups receive. Can take `private`, `internal` and `public` as a parameter. Default is `private`. |