zmq_send(3) manpage improved

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

doc/zmq_send.txt

@@ -28,6 +28,10 @@ Specifies that the message being sent is a multi-part message, and that further
 message parts are to follow. Refer to the section regarding multi-part messages
 below for a detailed description.
 
+The _zmq_msg_t_ structure passed to _zmq_send()_ is nullified during the call.
+If you want to send the same message to multiple sockets you have to copy it
+using (e.g. using _zmq_msg_copy()_).
+
 NOTE: A successful invocation of _zmq_send()_ does not indicate that the
 message has been transmitted to the network, only that it has been queued on
 the 'socket' and 0MQ has assumed responsibility for the message.