caution about zmq_msg_init_* functions added to the docs

doc/zmq_msg_init.txt

@@ -21,6 +21,10 @@ before receiving a message with _zmq_recv()_.
 CAUTION: Never access 'zmq_msg_t' members directly, instead always use the
 _zmq_msg_ family of functions.
 
+CAUTION: The functions _zmq_msg_init()_, _zmq_msg_init_data()_ and
+_zmq_msg_init_size()_ are mutually exclusive. Never initialize the same
+'zmq_msg_t' twice.
+
 
 RETURN VALUE
 ------------

doc/zmq_msg_init_data.txt

@@ -28,6 +28,10 @@ supplied to _zmq_msg_init_data()_.
 CAUTION: Never access 'zmq_msg_t' members directly, instead always use the
 _zmq_msg_ family of functions.
 
+CAUTION: The functions _zmq_msg_init()_, _zmq_msg_init_data()_ and
+_zmq_msg_init_size()_ are mutually exclusive. Never initialize the same
+'zmq_msg_t' twice.
+
 
 RETURN VALUE
 ------------

doc/zmq_msg_init_size.txt

@@ -25,6 +25,10 @@ _zmq_msg_init_size()_ shall not clear the message data.
 CAUTION: Never access 'zmq_msg_t' members directly, instead always use the
 _zmq_msg_ family of functions.
 
+CAUTION: The functions _zmq_msg_init()_, _zmq_msg_init_data()_ and
+_zmq_msg_init_size()_ are mutually exclusive. Never initialize the same
+'zmq_msg_t' twice.
+
 
 RETURN VALUE
 ------------