docs(form): provide a list of Angular's built-in validation tokens
docs(form): provide a list of Angular's built-in validation tokens

As requested by a top-rated Disqus comment: http://docs.angularjs.org/api/ng.directive:form.FormController#comment-655325797

Closes #5121

ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js,ammula88/angular.js