Clarify splitting {session,keyFetch}Token into id and reqHMACKey

Token is 64 bytes, which gets split into 32 byte id and 32 byte request HMAC key.  The Hawk id is the hex-string of id and the key that Hawk is using to HMAC is actually the UTF-8 bytes of the hex-string of the request HMAC key.  That surprised me.  We should clarify *exactly* what the id and the key are.