Add subgroups limitations to Pages docs
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Pages doesn't work with subgroups. We added this limitation to the Subsgroup doc, but not to Pages. This MR fill this gap.
EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2444