explain None can be returned

Lib/ssl.py

@@ -573,7 +573,9 @@ class SSLObject:
         return self._sslobj.cipher()
 
     def shared_ciphers(self):
-        """Return the ciphers shared by the client during the handshake."""
+        """Return the a list of ciphers shared by the client during the
+        handshake or None if this is not a valid server connection.
+        """
         return self._sslobj.shared_ciphers()
 
     def compression(self):