Document values of `MergeRequest.mergeStatus`

In GraphQL this is a String, in REST likewise.

In the model it is a defined state-machine, with the following states:

    state :preparing
    state :unchecked
    state :cannot_be_merged_recheck
    state :checking
    state :cannot_be_merged_rechecking
    state :can_be_merged
    state :cannot_be_merged

Defined at: https://gitlab.com/gitlab-org/gitlab/blob/5bf601065e7555f3993e75f59a48eabcf3d69023/app/models/merge_request.rb#L216-222

We should:

• Document these values in the REST API docs
• GQL: add MergeRequest.mergeStatusEnum field, with a generated enum type
• GQL: deprecate MergeRequest.mergeStatus (optional - there is no harm in leaving it around)