Doc & test that update_merge_request can set state

Update the documentation for Gitlab.update_merge_request to say that
Gitlab.update_merge_request can change a merge request's state.

Test that Gitlab.update_merge_request Gitlab.update_merge_request
can change a merge request's state.
Add merge_request_comments to get a MR's comments

Add a method merge_request_comments to retrieve a merge request's
comments.

Document and add spec tests for the new method.