Docs: Update admin docs
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Follow up from https://gitlab.com/gitlab-org/gitlab-ce/issues/39688
Adds general updates to admin docs
EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3751
Edited by Marcia Ramos