Documentation for `ListMergeRequests` should be updated

Created by: FlorianLoch

ListMergeRequests returns []*MergeRequest - which is tricky, as - according to GitLab's example and real-world experience - the response of the API is not fully populated.

That results in not fully populated instances of MergeRequest.

head_pipeline is an example for that.

I would therefore suggest to either create another type representing just the subset of fields that actually get returned or to at least state it in the docs. Because like this it's hard to tell whether a field is indeed empty/false/0 or whether it's just the zero value.