Complete `update` method with argument documentation.

Currently the documentation says that the `update` method doesn't take any arguments. This is untrue. The method take one argument of type **String** or **Date**.

The following block of code proves it.

```javascript
if(arguments && arguments.length && (typeof arguments[0] === 'string' || arguments[0] instanceof Date)) {
    date = arguments[0];
    fromArgs = true;
} else {
    date = this.isInput ? this.element.val() : this.element.data('date') || this.element.find('input').val();
}
```