Docs: permissions - update
Update https://docs.gitlab.com/ee/user/permissions.html: non-members cannot assign issues and MRs to anyone, only mention team members. Same for labels.
Follow up from gitlab-com/www-gitlab-com!8197 (comment 47055118)
cc/ @axil + @jramsay or @victorwu (not sure if this falls under "platform" or "discussions"
Edited by Marcia Ramos