Add guidance on the access of data referred to be id's in hook context.

[bvdh]

As is mentioned in #502 and #506, some guidance is required related to access of Resources that is referred to by id from the hook context.
I suggest that we add the following language to the Hook context and prefetch section in the spec.

If the CDS-Client supports pretech and/or provides a FHIR Server, the CDS-Client SHOULD provide FHIR Resources referenced by FHIR ids in context. As this behavior is optional, CDS-Server implementers need to be aware that not all CDS-Clients will support this behavior.