Document the property configuration of the auto-configured EntityManagerFactoryBuilder

[wilkinsona]

See #15318. We need to describe that some properties are applied to the auto-configured EntityManagerFactoryBuilder and that some are only applied in isolation when the builder's being used to create the entity manager factory.

[mbhave]

@wilkinsona I tried documenting this. Could you take a look and see if it's accurate and what you had in mind?

[wilkinsona]

LGTM, @mbhave. Thank you.