docs(ngNonBindable): document effect on the element's directives

The phrase "contents of the current DOM element" may be interpreted either as
inclusive of the DOM element's attributes or as exclusive of the attributes.
This situation concerns markup such as:

    <div ng-non-bindable ng-controller="MyController"></div>

In practice, AngularJS does not compile or bind attribute values for elements
which specify the `ng-non-bindable` directive. Extend the documentation to
definitely describe this behavior.

Closes #16338