Add branch variable expansion documentation

doc/ci/multi_project_pipelines.md

@@ -116,7 +116,8 @@ staging:
 ```
 
 Use a `project` keyword to specify full path to a downstream project. Use
-a `branch` keyword to specify a branch name.
+a `branch` keyword to specify a branch name. Variable expansion is supported in
+the `branch` property.
 
 GitLab will use a commit that is currently on the HEAD of the branch when
 creating a downstream pipeline.