Perhaps make it more consistent with document::element. It privately inherits from document::element, and (I think) does not expose everything document::element does. (Also, why is there a distinction?).

There is an artificial distinction between a bsoncxx::document::element and a bsoncxx::array::element with the latter missing some features of the former. This also makes it harder for code to just work with any bson element, regardless of what it is contained in.