docs(input): document NgModelController.$isEmpty parameters / return value

Closes angular#6224

Author: MathieuTricoire

src/ng/directive/input.js

@@ -1022,6 +1022,9 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * 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.
    */
   this.$isEmpty = function(value) {
     return isUndefined(value) || value === '' || value === null || value !== value;