Add cross references to Runners API
To close https://gitlab.com/gitlab-org/gitlab-ce/issues/28014, cross reference to API, showing that the API supports the options of gitlab-runner commands because gitlab-runner uses the API under the hood.
To close https://gitlab.com/gitlab-org/gitlab-ce/issues/28014, cross reference to API, showing that the API supports the options of gitlab-runner commands because gitlab-runner uses the API under the hood.