Docs: explain feature availability in GitLab.com
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- For https://gitlab.com/gitlab-org/gitlab-ce/issues/42631
- Follows https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16862
- Explains feature availability in GitLab.com tiers
EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4703
Edited by Marcia Ramos