Docs: Pipeline subscription needs owner permission

The form to add pipeline subscriptions
will fail to resolve the project path if the user
adding it lacks owner rights to the project
even if it is public

doc/ci/multi_project_pipelines.md

@@ -324,8 +324,9 @@ now trigger a pipeline on the current project's default branch. The maximum
 number of upstream pipeline subscriptions is 2 by default, for both the upstream and
 downstream projects. This [application limit](../administration/instance_limits.md#number-of-cicd-subscriptions-to-a-project) can be changed on self-managed instances by a GitLab administrator.
 
-The upstream project needs to be [public](../public_access/public_access.md) for
-pipeline subscription to work.
+The upstream project needs to be [public](../public_access/public_access.md)
+and the user must have [developer permissions](../user/permissions.md#project-members-permissions)
+for the upstream project.
 
 ## Downstream private projects confidentiality concern