specify that groups shall be UTF8

group being a `char *` is logically a text type, which needs an encoding.

Declare in the API that groups shall be UTF8-encoded,
matching the `zmq_msg_gets` API, which is the other user-facing `char *` API,
which has the same definition.

This allows bindings to provide text-type APIs,
which they cannot do if arbitrary bytes are allowed

doc/zmq_socket.txt

@@ -160,6 +160,7 @@ and each message sent by Radio sockets belong to a group.
 
 Groups are null terminated strings limited to 16 chars length (including null).
 The intention is to increase the length to 40 chars (including null).
+The encoding of groups shall be UTF8.
 
 Groups are matched using exact matching (vs prefix matching of PubSub).