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Update MR-related docs
- 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