doc: clarify child_process error behaviour

Clarify that an 'error' event may or may not be followed by an 'exit'
event and that it's not safe to make assumptions either way.

doc/api/child_process.markdown

@@ -36,6 +36,10 @@ Emitted when:
 2. The process could not be killed, or
 3. Sending a message to the child process failed for whatever reason.
 
+Note that the `exit`-event may or may not fire after an error has occured. If
+you are listening on both events to fire a function, remember to guard against
+calling your function twice.
+
 See also [`ChildProcess#kill()`](#child_process_child_kill_signal) and
 [`ChildProcess#send()`](#child_process_child_send_message_sendhandle).