Documentation review for "Add or remove blocking merge requests"

References

Merged MR that introduced documentation requiring review: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13506#note_194419379

Related issue(s): https://gitlab.com/gitlab-org/gitlab-ee/issues/9688

Further Details

This is a new feature that means an MR can prevent another MR from being merged. I've written some user documentation for it at https://docs.gitlab.com/ee/user/project/merge_requests/blocking_merge_requests.html that needs review.