Improved DQL's "new" operator documentation

I had to dig through the pull requests and issues to figure that out, so I believe it's best documented explicitly.

docs/en/reference/dql-doctrine-query-language.rst

@@ -509,6 +509,8 @@ And then use the ``NEW`` DQL keyword :
     $query = $em->createQuery('SELECT NEW CustomerDTO(c.name, e.email, a.city, SUM(o.value)) FROM Customer c JOIN c.email e JOIN c.address a JOIN c.orders o GROUP BY c');
     $users = $query->getResult(); // array of CustomerDTO
 
+Note that you cannot pass entities as constructor parameters for now.
+
 Using INDEX BY
 ~~~~~~~~~~~~~~