default-to-one-fetch-type in persistence-unit-defaults

`orm.xml` has this `persistence-unit-defaults` stuff which I don't completely understand the semantics of, but I suppose it could be possible to specify `default-to-one-fetch-type` there.

Start by reading the `orm.xml` semantics for `persistence-unit-defaults` and the existing handling of fetch types. Determine whether `default-to-one-fetch-type` belongs there and what its expected behavior should be; the issue is complete only after that design is agreed and reflected in the persistence specification or implementation.

Written by the indexing model from the issue text.