Test `fetch( getReference)` and update javadoc

[DavideD]

Looking at the javadoc it seems that this usecase should not be supported:

return sessionFactory.withSession(s -> s.fetch(s.getReference(Author.class, authorId))
                .chain(author -> Mutiny.fetch(author.getBooks())));

We should add a test to both test classes:

• https://github.com/hibernate/hibernate-reactive/blob/main/hibernate-reactive-core/src/test/java/org/hibernate/reactive/ReferenceTest.java
• integration-tests/bytecode-enhancements-it/src/test/java/org/hibernate/reactive/it/ReferenceBETest.java