Added usage of eventPayload in the event handler.

doc/base_api.md

@@ -100,6 +100,15 @@ this.after('initialize', function() {
 });
 ```
 
+Example of `handler` having the `ev` and `data` parameters that can be used
+to receive data from the component's optional `trigger` parameter `eventPayload`.
+
+```js
+this.on(document, 'dataSent', function (ev, data) {
+  alert('Message sent: ' + data.msg);
+});
+```
+
 Example of `handler` being an object that maps event targets to callbacks.
 This is effectively event delegation; selector values are resolved, at event
 time, by keying into the `attr` property of the component.