Update RDF-star semantics document

Hi,
after our most recent discussions, I have updated the RDF-star semantics document <https://github.com/w3c/rdf-star-wg/wiki/RDF%E2%80%90star-semantics:-option-3>.
More specifically, I think it may be worthwhile to have some feedback on the section “Best practices and well-formedness”.
cheers
—e.