Modernize and clarify subprotocol sections

This branch has editorial changes to improve the clarity of our description of subprotocol-based versioning in the Tor protocol. In our current vocab, we don't want to refer to "Relay=3" as a "version", since that implies that "Relay=4" is a "greater version".

It also adopts the "subprotocols are flags" section of proposal 346.

Part of #317 (closed)