title: Customized attribute for each collection checkbox in symfony2 form
body: <p>I have a class with title and description attributes and a have created a collection form with multiple checkboxes.</p>
  
  <p>Can i set as title attribute for each checkbox the description of entity?</p>
  
  <p>The class:</p>
  
  <pre><code>class Carpeta {   
      private $id;
      private $titulo;
      private $descripcion;
  [...]
  }
  </code></pre>
  
  <p>The Form:</p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
      {
  [...]
  $builder-&gt;add(
              'carpetas',
              'entity',
              array(
                  'label' =&gt; 'str_documentacion_necesaria_general',
                  'multiple' =&gt; true,
                  'expanded' =&gt; true,
                  'class' =&gt; 'Itbid\DirectorioBundle\Entity\Carpeta',
                  'attr'=&gt; array('class'=&gt;'span12'),
                  'choices' =&gt; $this-&gt;em-&gt;getRepository('DirectorioBundle:Carpeta')-&gt;findAll()
              )
          );
  [...]
  }
  </code></pre>
  
  <p>The View:</p>
  
  <pre><code>[...]
  {{ form_widget(form.carpetas) }}
  [...]
  </code></pre>