Explicit client registration: Mention that the OP must check the "aud" of the received EC

[vdzhuvinov]

Mention in section 12.2.2.1. that OP needs to verify the "aud" claim in the received Entity Statements - that the claim is present and it has a single value that is the OP's Entity Identifier.

This check is required to prevent the misuse of an RP's EC published at its well-known endpoint. In a federation that supports both methods of client registration - automatic and explicit, if the "aud" check isn't made by the OP, one could download the RP's EC and use it to trigger its registration at the OP.