Cross-reference back to attention request docs

Let's explain what attention requests are, so if someone comes to the
quick actions page first, they can get answers.

doc/user/project/quick_actions.md

@@ -55,6 +55,7 @@ threads. Some quick actions might not be available to all subscription tiers.
 | `/assign me`                                                                                     | **{check-circle}** Yes | **{check-circle}** Yes | **{dotted-circle}** No | Assign yourself.                                                                                                                                                                                                                                                                                                                                                                                                                                          |
 | `/assign_reviewer @user1 @user2` or `/reviewer @user1 @user2` or `/request_review @user1 @user2` | **{dotted-circle}** No | **{check-circle}** Yes | **{dotted-circle}** No | Assign one or more users as reviewers.                                                                                                                                                                                                                                                                                                                                                                                                                    |
 | `/assign_reviewer me` or `/reviewer me` or `/request_review me`                                  | **{dotted-circle}** No | **{check-circle}** Yes | **{dotted-circle}** No | Assign yourself as a reviewer.                                                                                                                                                                                                                                                                                                                                                                                                                            |
+| `/attention @user1`                                                                                 | **{dotted-circle}** No | **{check-circle}** Yes | **{dotted-circle}** No | [Request attention](merge_requests/index.md#request-attention-to-a-merge-request) to a merge request from a user. |
 | `/award :emoji:`                                                                                 | **{check-circle}** Yes | **{check-circle}** Yes | **{check-circle}** Yes | Toggle emoji award.                                                                                                                                                                                                                                                                                                                                                                                                                                       |
 | `/child_epic <epic>`                                                                             | **{dotted-circle}** No | **{dotted-circle}** No | **{check-circle}** Yes | Add child epic to `<epic>`. The `<epic>` value should be in the format of `&epic`, `group&epic`, or a URL to an epic ([introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/7330) in GitLab 12.0).                                                                                                                                                                                                                                                   |
 | `/clear_health_status`                                                                           | **{check-circle}** Yes | **{dotted-circle}** No | **{dotted-circle}** No | Clear [health status](issues/managing_issues.md#health-status) ([introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/213814) in GitLab 14.7).                                                                                                                                                                                                                                                                                                       |