Tweaking extensions text

Two small changes here.  First, adding unnegotiated semantics to reserved bits in existing frames feels like a REALLY bad idea, since you're almost guaranteed to have collisions, so I'm suggesting we remove that as something that extensions can do unannounced.  Second, we permit negotiated changes to existing semantics, so I'm suggesting that instead of saying only DATA will ever be flow controlled, we say that making anything else flow-controlled is a changed semantic that has to be negotiated.