Group CI/CD settings owners only settings documentation

@godfat in https://gitlab.com/gitlab-org/gitlab-ce/issues/46671

@grzesiek Thanks! I didn't dig deep about this decision, but I guess we'll also need to make this clear on the documentation, particularly in https://docs.gitlab.com/ee/user/permissions.html#group-members-permissions maybe like adding a section for Manage CI/CD. Do you know if we have an issue for that?