No more labeled nodes

Apart from the other changes I've made, my draft of the syntax doc now
does not use phrases like "nodes are labeled with URI-refs ...".  The
replacement text is "nodes are identified by URI-refs, ..."  etc. and
so on.

I also changed the words around to make clear the RDF graph is a set
of triples (this is duplicating words in rdf-concepts somewhat;
something we're working on).

Pat: I hope that's better, think you asked for it.

Dave