docs(NgModelController): clarify the value parameter for $isEmpty

src/ng/directive/input.js

@@ -1814,17 +1814,18 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * @name ngModel.NgModelController#$isEmpty
    *
    * @description
-   * This is called when we need to determine if the value of the input is empty.
+   * This is called when we need to determine if the value of an input is empty.
    *
    * For instance, the required directive does this to work out if the input has data or not.
+   *
    * The default `$isEmpty` function checks whether the value is `undefined`, `''`, `null` or `NaN`.
    *
    * You can override this for input directives whose concept of being empty is different to the
    * default. The `checkboxInputType` directive does this because in its case a value of `false`
    * implies empty.
    *
-   * @param {*} value Reference to check.
-   * @returns {boolean} True if `value` is empty.
+   * @param {*} value The value of the input to check for emptiness.
+   * @returns {boolean} True if `value` is "empty".
    */
   this.$isEmpty = function(value) {
     return isUndefined(value) || value === '' || value === null || value !== value;