Clarify that both Order and User extend Persistent in example in section 5.2.4

The example doesn't specify that either Order or User extend Persistent. This must be assumed by the reader.

Also, one other thing: when a reader is reading the spec, they come across the following sentence: "For example, Dao is eligible for injection to any injection point of type..." and it's not immediately clear what Dao it is talking about, since Dao is defined later (currently not even on the same page in the PDF). It would be much clearer if the Dao was defined earlier.