Document that :enabled and :disabled represent the boolean value of a property

They are different from "[disabled]", which would detect the existence of a disabled property regardless of its value.

In brief:

```
"enabled": function( elem ) {
    return elem.disabled === false;
},
"disabled": function( elem ) {
    return elem.disabled === true;
},
```

Please see http://bugs.jquery.com/ticket/12295 for more information
