Update details about when MR pipelines run

doc/ci/pipelines/merge_request_pipelines.md

@@ -25,10 +25,11 @@ Branch pipelines:
 Merge request pipelines:
 
 - Run when you:
-  - Create a new merge request.
+  - Create a new merge request from a source branch with one or more commits.
   - Push a new commit to the source branch for a merge request.
   - Select **Run pipeline** from the **Pipelines** tab in a merge request. This option
-    is only available when merge request pipelines are configured for the pipeline.
+    is only available when merge request pipelines are configured for the pipeline
+    and the source branch has at least one commit.
 - Do not run by default. The jobs in the CI/CD configuration file [must be configured](#prerequisites)
   to run in merge request pipelines.
 - Have access to [more predefined variables](#available-predefined-variables).