Document safe way to access the information of an element with a widget bound to it

[FagnerMartinsBrack]

As discussed on http://bugs.jqueryui.com/ticket/8908, element.is( ":data(namespace-widget)" ) should be documented as a safe way to detect if a widget is being used in a given element.

Suggestion 1: It can be written under "Additional Notes" of each component like "Detect if the widget is active on the element: $( ".selector" ).is( ":data(namespace-widget)" )"

Suggestion 2: It can be written along with the destroy() method in Code examples: like:

Code Examples:
Detect if the widget is active on the element:
$( ".selector" ).is( ":data(namespace-widget)" );

[tjvantoll]

This is likely going to be addressed by jquery/jquery-ui#902.

[scottgonzalez]

Yup, but for old versions, we should document the selector approach.

[tjvantoll]

A section about this was added in the widget's factory's docs, but we still want something on the individual widget's pages. Reopening this so I remember to do that.

[jzaefferer]

Once the instance method landed, we also need to update the new Instance section.

[tjvantoll]

@jzaefferer See #114.

I think that pull covers most of this issue. The one thing remaining is we don't have a means of determining whether an element has a given widget bound to it documented on each widget's page as the OP suggested. Currently it's only documented on the widget factory's main API page.