1. Added specific details on DATA_V2/peer-id/float support.
2. For AEAD mode, emphasized that the leading 8 bytes (4 bytes for
DATA_V2/peer-id and 4 for packet ID) is all included in the AD.
3. Added specific details on protocol negotiation where the client
indicates protocol extension availability with IV_x parameters
in the peer info string, and the server responds by pushing
directives to the client to enable the feature.
4. Added "TCP nonlinear mode" section, a new protocol extension
that is needed by multithreaded TCP servers.
