Kubernetes cluster > Adding existing cluster > Unmanaged cluster > Project namespace > Document

When a user is adding an existing cluster and chooses for it to be un-managed by GitLab, they need to provide a project namespace in order for use to detect it and use it for deploy boards, pod logs, and Web terminals. We don't have this documented on our cluster page https://docs.gitlab.com/ee/user/project/clusters/. We should add this information so its clear to users what the field is for.