Add core-only mark to settings API documentation

This MR is a follow-up to !20674, specifically https://gitlab.com/gitlab-org/gitlab/merge_requests/20674#note_267369748

doc/api/api_resources.md

@@ -129,7 +129,7 @@ The following API resources are available outside of project and group contexts
 | [Projects](projects.md)                           | `/users/:id/projects` (also available for projects)                     |
 | [Runners](runners.md)                             | `/runners` (also available for projects)                                |
 | [Search](search.md)                               | `/search` (also available for groups and projects)                      |
-| [Settings](settings.md)                           | `/application/settings`                                                 |
+| [Settings](settings.md) **(CORE ONLY)**           | `/application/settings`                                                 |
 | [Statistics](statistics.md)                       | `/application/statistics`                                               |
 | [Sidekiq metrics](sidekiq_metrics.md)             | `/sidekiq`                                                              |
 | [Suggestions](suggestions.md)                     | `/suggestions`                                                          |

doc/api/settings.md

-# Application settings API
+# Application settings API **(CORE ONLY)**
 
 These API calls allow you to read and modify GitLab instance
 [application settings](#list-of-settings-that-can-be-accessed-via-api-calls)