The purpose of DocumentReference is to act as an indexing resource. It's supposed to "point" to the document, but it would defeat the purpose of DocumentReference if the document itself were contained in the resource. (Particularly given that the "attachment" is identified as being part of "summary".) Suggest adding an invariant that prohibits DocumentReference.content.attachment.data.

(Apparently Argonaut is looking at sending the document data inside the DocumentReference - this would force them to use a URL that points to a contained Binary (less ideal) or a remote Binary (better), but in either case at least we'd avoid the 'summary' problem.)