Problem: documentation does not warn about ZMQ_EVENT_ALL and new events

Solution: add a note pointing out that when new events are added the
ZMQ_EVENT_ALL mask will start returning them, so applications that
can't handle that transparently should not use it and instead pick
only the event types they want.
Creating a new event type and having monitors with ZMQ_EVENT_ALL
return them will not be considered a backward incompatible change
even though the flow of events will change.

doc/zmq_socket_monitor.txt

@@ -23,7 +23,10 @@ your own 'ZMQ_PAIR' socket, and connect that to the endpoint.
 
 The 'events' argument is a bitmask of the socket events you wish to
 monitor, see 'Supported events' below. To monitor all events, use the
-event value ZMQ_EVENT_ALL.
+event value ZMQ_EVENT_ALL. NOTE: as new events are added, the catch-all
+value will start returning them. An application that relies on a strict
+and fixed sequence of events must not use ZMQ_EVENT_ALL in order to
+guarantee compatibility with future versions.
 
 Each event is sent as two frames. The first frame contains an event
 number (16 bits), and an event value (32 bits) that provides additional