chore: add recommedation about public key used in the jwt proof

This PR aims to remark an important aspect about the unlinkability of the credential through the holder key binding.

docs/en/pid-eaa-issuance.rst

@@ -322,6 +322,10 @@ The ``client_assertion`` is signed using the private key that is created during
 
     **PID/(Q)EAA Credential Schema and Status registration:** The PID/(Q)EAA Provider MUST register all the issued Credentials for their later revocation, if needed.
 
+.. note::
+
+    **It is RECOMMENDED that the public key contained in the ``jwt_proof`` be specifically generated for the requested Credential (fresh cryptographic key) to ensure that different issued Credentials do not share the same public key, thereby remaining unlinkable to each other.
+
 .. code-block::
 
   POST /credential HTTP/1.1