Improve language around holder binding

[decentralgabe]

Currently, Section 5.1.3 with respect to the cnf property states:

If cnf is present in the JOSE Header or the JWT Claims , a verifier MAY use this parameter to identify a proof-of-possession key in the manner described in [RFC7800] for use in the verification process.

Related to this comment on the VC Data Model, I believe we should improve upon this language, especially for presentations, perhaps even to go as far as to require usage of the cnf property for presentations.

Another consideration is to speak to how the cnf property works with multiple subjects.