Merge branch 'document-merge-request-event-type-variable' into 'master'

Document `CI_MERGE_REQUEST_EVENT_TYPE`

Closes #37482

See merge request gitlab-org/gitlab!22145

doc/ci/variables/predefined_variables.md

@@ -79,6 +79,7 @@ future GitLab releases.**
 | `CI_MERGE_REQUEST_TARGET_BRANCH_NAME`         | 11.6   | all    | The target branch name of the merge request if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` is used and the merge request is created.                                                                                                                                           |
 | `CI_MERGE_REQUEST_TARGET_BRANCH_SHA`          | 11.9   | all    | The HEAD SHA of the target branch of the merge request if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` is used, the merge request is created, and the pipeline is a [merged result pipeline](../merge_request_pipelines/pipelines_for_merged_results/index.md). **(PREMIUM)**   |
 | `CI_MERGE_REQUEST_TITLE`                      | 11.9   | all    | The title of the merge request if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` is used and the merge request is created.                                                                                                                                                        |
+| `CI_MERGE_REQUEST_EVENT_TYPE`                 | 12.3   | all    | The event type of the merge request, if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Can be `detached`, `merged_result` or `merge_train`. |
 | `CI_NODE_INDEX`                               | 11.5   | all    | Index of the job in the job set. If the job is not parallelized, this variable is not set.                                                                                                                                                                                                                                                                 |
 | `CI_NODE_TOTAL`                               | 11.5   | all    | Total number of instances of this job running in parallel. If the job is not parallelized, this variable is set to `1`.                                                                                                                                                                                                                                    |
 | `CI_PAGES_DOMAIN`                             | 11.8   | all    | The configured domain that hosts GitLab Pages.                                                                                                                                                                                                                                                                                                             |