Add documentation for approval_settings endpoint

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We seem to be missing documentation on https://docs.gitlab.com/ee/api/merge_request_approvals.html for the approval_settings endpoint, e.g. /api/v4/projects/8/merge_requests/2/approval_settings. I noticed this as part of !39237 (merged) and #10294 (closed).

This should be fairly simple to add as the fields are largely similar to other endpoints on the page already.

/cc @m_gill