Merge branch 'docs-merge-train-retry-details' into 'master'

Update details about retrying merge trains

See merge request gitlab-org/gitlab!50417

doc/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md

@@ -177,9 +177,13 @@ for more information.
 
 ### Merge Train Pipeline cannot be retried
 
-A Merge Train pipeline cannot be retried because the merge request is dropped from the merge train upon failure. For this reason, the retry button does not appear next to the pipeline icon.
+When a pipeline for merge trains fails the merge request is dropped from the train and the pipeline can't be retried.
+Pipelines for merge trains run on the merged result of the changes in the merge request and
+the changes from other merge requests already on the train. If the merge request is dropped from the train,
+the merged result is out of date and the pipeline can't be retried.
 
-In the case of pipeline failure, you should [re-enqueue](#add-a-merge-request-to-a-merge-train) the merge request to the merge train, which then initiates a new pipeline.
+Instead, you should [add the merge request to the train](#add-a-merge-request-to-a-merge-train)
+again, which triggers a new pipeline.
 
 ### Unable to add to merge train with message "The pipeline for this merge request failed."