Spec: improve contextual description of EXTEND->CREATE / CREATED->EXTENDED handling and payloads

catalyst noted in #tor-dev that 5.1.2 (EXTEND and EXTENDED) says nothing explicitly about an OR's (relays receiving an EXTEND or EXTEND2 cell) responsibility to send a corresponding CREATE/CREATE2 cell to the requested next node.

catalyst, asn, nickm, and I chatted a bit about it. I bought up that 5.3 mentions an OR's role of translating EXTEND -> CREATE and CREATED -> EXTENDED and suggested we might reference that section from 5.1 and 5.1.2.

It's probably enough to do that, but if some other change makes more sense (when actually putting together the wording), do that.