Widget method examples

Done except for one issue. The common methods - `destroy()`, `disable()`, `enable()`, `option()`, and `widget()` - are in separate files, for example [widget-method-destroy.xml](https://github.com/jquery/api.jqueryui.com/blob/cd515b79d0ec792c1d7610b13b7938d1143d48eb/includes/widget-method-destroy.xml).

For widgets, the xsl adds a code example showing the plugin invocation, but that won't work here since we need an example with the *instance* invocation. So the choices are:

* Explicitly list those methods in jQuery.widget.xml - which would be some duplication. 
* xsl magic?

@scottgonzalez Thoughts?