The JPA 2.0 specification section 7.9.1 states the following requirements:
7.9.1 Container Responsibilities
For the management of a transaction-scoped persistence context, if there is no EntityManager alreadyassociated with the JTA transaction:
- The container creates a new entity manager by calling EntityManagerFactory.createEntityManager when the first invocation of an entity manager with PersistenceContextType.TRANSACTION occurs within the scope of a business method executing in the JTA transaction.
- After the JTA transaction has completed (either by transaction commit or rollback), The container closes the entity manager by calling EntityManager.close. 
... see JPA 2.0 spec for ommitted text that is not relevant to this discussion ...
 The container may choose to pool EntityManagers: it instead of creating and closing in each case, it may acquire one from its pool and call clear() on it.
The following is thus wrong: http://www.datanucleus.org/products/accessplatform/jpa/emf.html
Description The JPA spec (3.11) says that a "container-managed" EntityManager should not be closed (by calling close()). Some JavaEE containers ignore this, and are non-compliant as a result but you can ignore such calls if you set this persistence property to true.
Range of Values false | true