docs(ngModel): use arguments.length instead of angular.isDefined(newName) to distinguish getter/setter usage

Closes #11604

Author: yankee42

src/ng/directive/ngModel.js

@@ -996,10 +996,11 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
            var _name = 'Brian';
            $scope.user = {
              name: function(newName) {
-               if (angular.isDefined(newName)) {
-                 _name = newName;
-               }
-               return _name;
+              // Note that newName can be undefined for two reasons:
+              // 1. Because it is called as a getter and thus called with no arguments
+              // 2. Because the property should actually be set to undefined. This happens e.g. if the
+              //    input is invalid
+              return arguments.length ? (_name = newName) : _name;
              }
            };
          }]);
@@ -1217,7 +1218,11 @@ var DEFAULT_REGEXP = /(\s+|^)default(\s+|$)/;
           var _name = 'Brian';
           $scope.user = {
             name: function(newName) {
-              return angular.isDefined(newName) ? (_name = newName) : _name;
+              // Note that newName can be undefined for two reasons:
+              // 1. Because it is called as a getter and thus called with no arguments
+              // 2. Because the property should actually be set to undefined. This happens e.g. if the
+              //    input is invalid
+              return arguments.length ? (_name = newName) : _name;
             }
           };
         }]);