docs: process.on('exit') receives exit code

The fact that the "exit" event passes the exit code as an argument
as omitted from the documentation. This adds the explanation and
augments the example code to show that.

doc/api/process.markdown

@@ -11,15 +11,16 @@ It is an instance of [EventEmitter][].
 Emitted when the process is about to exit.  This is a good hook to perform
 constant time checks of the module's state (like for unit tests).  The main
 event loop will no longer be run after the 'exit' callback finishes, so
-timers may not be scheduled.
+timers may not be scheduled.  The callback takes one argument, the code the
+process is exiting with.
 
 Example of listening for `exit`:
 
-    process.on('exit', function() {
+    process.on('exit', function(code) {
       setTimeout(function() {
         console.log('This will not run');
       }, 0);
-      console.log('About to exit.');
+      console.log('About to exit with code:', code);
     });
 
 ## Event: 'uncaughtException'