docs(ngDisabled): clarify the explanation of attributes & interpolation

Closes #11032
Closes #11133

Author: Narretz

src/ng/directive/attrs.js

@@ -159,20 +159,23 @@
  *
  * @description
  *
- * We shouldn't do this, because it will make the button enabled on Chrome/Firefox but not on IE8 and older IEs:
+ * This directive sets the `disabled` attribute on the element if the
+ * {@link guide/expression expression} inside `ngDisabled` evaluates to truthy.
+ *
+ * A special directive is necessary because we cannot use interpolation inside the `disabled`
+ * attribute.  The following example would make the button enabled on Chrome/Firefox
+ * but not on older IEs:
+ *
  * ```html
- * <div ng-init="scope = { isDisabled: false }">
- *  <button disabled="{{scope.isDisabled}}">Disabled</button>
+ * <div ng-init="isDisabled = false">
+ *  <button disabled="{{isDisabled}}">Disabled</button>
  * </div>
  * ```
  *
- * The HTML specification does not require browsers to preserve the values of boolean attributes
- * such as disabled. (Their presence means true and their absence means false.)
+ * This is because the HTML specification does not require browsers to preserve the values of
+ * boolean attributes such as `disabled` (Their presence means true and their absence means false.)
  * If we put an Angular interpolation expression into such an attribute then the
  * binding information would be lost when the browser removes the attribute.
- * The `ngDisabled` directive solves this problem for the `disabled` attribute.
- * This complementary directive is not removed by the browser and so provides
- * a permanent reliable place to store the binding information.
  *
  * @example
     <example>
@@ -191,7 +194,7 @@
  *
  * @element INPUT
  * @param {expression} ngDisabled If the {@link guide/expression expression} is truthy,
- *     then special attribute "disabled" will be set on the element
+ *     then the `disabled` attribute will be set on the element
  */