Clarified up and downsides of $collection->toArray()

manual/en/working-with-objects.txt

@@ -457,6 +457,10 @@ and encapsulating all the details inside the classes can be challenging.
         }
     }
 
+This will however always initialize the collection, with all the performance penalties given the size. In
+some scenarios of large collections it might even be a good idea to completely hide the read access behind
+methods on the EntityRepository.
+
 There is no single, best way for association management. It greatly depends on the requirements of your concrete
 domain model as well as your preferences.