Fix GROUP BY example in documentation (GROUP BY object instance does not work)

Chapter 11 of the Hibernate documentation shows three HQL examples where GROUP BY is used with an object instance. E.g.

select cat, count( elements(cat.kittens) )
from eg.Cat cat group by cat

This does not work. Refer to the following forum posts for details:

http://forum.hibernate.org/viewtopic.php?t=953716
http://forum.hibernate.org/viewtopic.php?t=950514