Document ChildProcess exit/close event difference

doc/api/child_process.markdown

@@ -35,8 +35,16 @@ normally, `code` is the final exit code of the process, otherwise `null`. If
 the process terminated due to receipt of a signal, `signal` is the string name
 of the signal, otherwise `null`.
 
+Note that the child process stdio streams might still be open.
+
 See `waitpid(2)`.
 
+### Event: 'close'
+
+This event is emitted when the stdio streams of a child process have all
+terminated.  This is distinct from 'exit', since multiple processes
+might share the same stdio streams.
+
 ### Event: 'disconnect'
 
 This event is emitted after using the `.disconnect()` method in the parent or