ISPN-4364 Add documentation for Entry Retriever

* Added documentation for retriever
* Also made sure that filter package is public as it should be

https://issues.jboss.org/browse/ISPN-4364
Basically is trying to say you can use the Iterable to create multiple iterators that are invoked when created (so you can create multiple iterators at different times).  Let me try to clarify it more.