Add event.preventDefault documentation to javascript.md

docs/4.0/getting-started/javascript.md

@@ -36,7 +36,7 @@ $(document).off('.alert.data-api')
 
 Bootstrap provides custom events for most plugins' unique actions. Generally, these come in an infinitive and past participle form - where the infinitive (ex. `show`) is triggered at the start of an event, and its past participle form (ex. `shown`) is triggered on the completion of an action.
 
-All infinitive events provide [`preventDefault()`](https://developer.mozilla.org/en-US/docs/Web/API/Event/preventDefault) functionality. This provides the ability to stop the execution of an action before it starts.
+All infinitive events provide [`preventDefault()`](https://developer.mozilla.org/en-US/docs/Web/API/Event/preventDefault) functionality. This provides the ability to stop the execution of an action before it starts. Returning false from an event handler will also automatically call `preventDefault()`
 
 {% highlight js %}
 $('#myModal').on('show.bs.modal', function (e) {