Update COSE_Encrypt scope and future extension note

Towards #75:

> it would be nice to clarify why COSE_Encrypt is excluded and whether future extensions are possible.

Author: achamayou

draft-ietf-cose-hash-envelope.md

@@ -180,7 +180,7 @@ Note that when using a pre-hash algorithm, the algorithm SHOULD be registered in
 
 ## COSE_Encrypt
 
-Only COSE_Sign/COSE_Sign1 and COSE_Mac/COSE_Mac0 are in scope for this document. COSE_Encrypt/COSE_Encrypt0 is out of the scope of this document.
+Only COSE_Sign/COSE_Sign1 and COSE_Mac/COSE_Mac0 are in scope for this document. COSE_Encrypt/COSE_Encrypt0 is out of the scope of this document, because there is no known use case for it. It may be covered by a future extension, which would need to address whether the hash function is applied before or after encryption, and clarify privacy considerations arising from that decision.
 
 ## Payload Verification