Clarify CI_MERGE_REQUEST_APPROVED value when there is no approval rule
When a merge request has no approval rules, and no approval given, the variable `CI_MERGE_REQUEST_APPROVED` is `true`. This should be clarified in the documentation. Also see: - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64677#note_610079324 - https://gitlab.com/gitlab-org/gitlab/-/issues/334352#note_618215158
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