Add note about the property attribute (Property Accessor)

I think we must document the fact that propety is a path which is evaluted by the PropertyAccessor component.

reference/forms/types/entity.rst

@@ -96,6 +96,23 @@ This is the property that should be used for displaying the entities
 as text in the HTML element. If left blank, the entity object will be
 cast into a string and so must have a ``__toString()`` method.
 
+Note: ``property`` is the property path used to display the option. So you 
+can use anything supported by the :doc:`PropertyAccessor component</components/property_access/introduction>`
+
+Usage sample:
+
+    $builder->add('gender', 'entity', array(
+        'class' => 'MyBundle:Gender',
+        'property' => 'translations[en].name',
+        'query_builder' => function(EntityRepository $er) {
+            return $er->createQueryBuilder('g')
+               ->join('g.translations', 't')
+               ->where('t.locale = :locale')
+               ->orderBy('t.name', 'ASC')
+               ->setParameter('locale', 'en');
+        },
+    ));
+
 group_by
 ~~~~~~~~