Follow-up: Document Merge Request settings at the group level

The following discussion from !102863 (merged) should be addressed:

• @eread started a discussion: (+4 comments)

  @luzhiyuan.deer I've reviewed the UI text. For documentation, we will want a new section near: https://docs.gitlab.com/ee/user/group/manage.html#group-merge-request-approval-settings.

  Thanks!