Adding sorting to search results.

It's not clear what kind of thing this field refers to - is it the name of a field in the mongodoc entity struct, for example, or a field as stored in elastic search, or a field as stored in mongo?

Looking at the implementation, it seems to be none of the above, but the name as specified in the API. Perhaps it might be best to enumerate the allowed fields, and translate to those values inside v4. That way it becomes more obvious which fields are currently supported.