Update MR-related docs (!12875) · Merge requests · GitLab.org / GitLab FOSS · GitLab

Porting changes introduced by https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2350 to CE
Copyedit https://docs.gitlab.com/ee/user/project/merge_requests/index.html

For:

Original EE Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/2703
Port to CE: https://gitlab.com/gitlab-org/gitlab-ee/issues/2703