Define authentication for Transport

<note that the definition from JDF-389 and the following from PTK-69 should be merged>
5.2 Authentication
Whereas encryption allows for secure communication in the internet by hiding the contents of transactions from third parties, authentication is used to verify that the sender of a message is actually the entity that it claims to be. Authentification is therefore extremely important when fulfilling contracts. RFC 6750 provides methods to verify the identity of a message sender. PrintTalk defines no proprietary authentication methods but it is emphasized that authentication is important in all business transactions.