Clarify pipeline subscription namespace detail

doc/ci/multi_project_pipelines.md

@@ -266,7 +266,8 @@ You can trigger a pipeline in your project whenever a pipeline finishes for a ne
 tag in a different project:
 
 1. Go to the project's **Settings > CI / CD** page, and expand the **Pipeline subscriptions** section.
-1. Enter the path to the project you want to subscribe to.
+1. Enter the project you want to subscribe to, in the format `<namespace>/<project>`.
+   For example, if the project is `https://gitlab.com/gitlab-org/gitlab`, use `gitlab-org/gitlab`.
 1. Click subscribe.
 
 Any pipelines that complete successfully for new tags in the subscribed project