Adds documentation for stage button

Includes missing documentation for 'Play all manual' builds a single
stage.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/28741

doc/ci/pipelines.md

@@ -330,6 +330,19 @@ GitLab provides API endpoints to:
   - [Triggering pipelines through the API](triggers/README.md).
   - [Pipeline triggers API](../api/pipeline_triggers.md).
 
+### Start multiple manual actions in a stage
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27188) in GitLab 11.11.
+
+Multiple manual actions in a single stage can be started at the same time using the "Play all manual" button.
+Once the user clicks this button, each individual manual action will be triggered and refreshed
+to an updated status.
+
+This functionality is only available:
+
+- For users with at least Developer access.
+- If the the stage contains [manual actions](#manual-actions-from-pipeline-graphs).
+
 ## Security on protected branches
 
 A strict security model is enforced when pipelines are executed on