Bug: fragref does not allow keyref

As I've always understood these elements, they do not make sense when used
to reference a fragment or synnote in another diagram. I know that the
tools we use to render syntax diagrams in IBM do not allow these to
reference elements in other diagrams.

The DITA 1.1 langspec for fragref/@href says that the target fragment
"should" be in the same diagram. I have always viewed this as a "must". The
purpose of