### Problem to solve GitLab provides [quick actions](https://docs.gitlab.com/ee/user/project/quick_actions.html) such as `/merge` so that I can merge a merge request that I have finished reviewing. When a project has [merge trains](https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/) enabled the `/merge` quick action ignores the merge trains and merges the branch directly, which seems like unexpected behavior. ### Further details This can be confusing/ unexpected behavior when you have merge trains enabled and use quick actions. ### Proposal The `/merge` should either start or add to the merge train if merge trains are enabled. We can even update the tooltip to the user to be clear what `/merge` is going to do, for example for "Merge when Pipeline succeeds" we have the following message:  Helper text should be as follows: * No existing merge train, and the MR's pipeline has not yet finished * Suggestion: **/merge** *Start a merge train when the pipeline succeeds* * No existing merge train, and the MR's pipeline has finished * Suggestion: **/merge** *Start a merge train* * A merge train is already in progress, and the MR's pipeline has not yet finished * Suggestion: **/merge** *Add to merge train when the pipeline succeeds* * A merge train is already in progress, and the MR's pipeline has finished * Suggestion: **/merge** *Add to merge train* ### Weighting: - [ ] BE: Extend Merge Request API `merge` endpoint to allow users to choose Auto Merge Method. - [ ] BE: Make `AutoMergeService` flexible to accept `update` at `execute` interface. - [ ] FE: Show helper text for slash command according to the most recommended auto-merge-strategy (BE should already have exposed it in widget.json) - [ ] Publish the feature (Documentation and Feature Flag Removal)