title: how to add a class to symfony widget root element
body: <p>i was able to add a class to the form itself and for example to a form_row like you can see on the screenshot by adding a second parameter to form_start/form_row and so on. but i was not able to find a solution to add a class to the parent of div.choice-box which already has the class form-group. whats the correct solution right here?</p>
  
  <p><a href="https://i.stack.imgur.com/QNBSt.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QNBSt.png" alt="symfony form output"></a></p>
  
  <p>thanks guys</p>