Update docs of `detailed_merge_status`

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What does this MR do and why?

Update document https://docs.gitlab.com/ee/api/merge_requests.html#merge-status.

The fields description in this document are inconsistent with the code, such as the missing commits_status field. The current MR updates them.

• Related to https://jihulab.com/gitlab-cn/gitlab/-/issues/4406+
• Related to Introduce merge check for merge request approva... (#444459 - closed) (feature issue for the item behind a feature flag)
• Related to [Feature flag] Rollout of `policy_mergability_c... (#473704) (rollout issue for the feature flag)

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