[noise] minor underspecification

[Justin Cormack]

5.3 says

Calls MixHash() once for each public key listed in the pre-messages
from handshake_pattern, with the specified public key as input (see
Section 7 for an explanation of pre-messages). If both initiator and
responder have pre-messages, the initiator's public keys are hashed
first.

It doesnt mention what order to call MixHash if there are both
ephemeral and static keys in the premessage on the same side (possible
in unusual fallback cases).

Justin