Update CE docs

Update CE docs with revisions made in:

• MRs: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2350 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12875 + https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2433
• Issues: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2330 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12881
• Issue Boards: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2296 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12543
• Groups: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2385 => https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12888 + https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2442

For https://gitlab.com/gitlab-org/gitlab-ee/issues/2703