withNewSession behavior is inconsistent with the documentation and seems contrary to it's purpose

The docs for `withNewSession` say (emphasis added):

> Provides a way to execute code within the context of a new Hibernate session which *shares the same transactional (JDBC Connection)* resource as the currently bound session.

However, several comments by Graeme and observed behavior show that it is now opening a new connection by design:

https://github.com/grails/grails-core/issues/10599#issuecomment-296542627
https://github.com/grails/grails-core/issues/10503#issuecomment-283313398

It used to be the case that you needed to use `withNewSession` to isolate certain updates/flushes from the outer session. For instance, it was common to use it in a custom validator to prevent polluting or accidentally flushing the outer session. It was also common to use it in the `before*` type callback methods, if querying data, to prevent accidental flushes.

Some of that is remedied by using `FlushMode.COMMIT` as the default, but it still seems like `withNewSession` can't really be used that way at all anymore.

If it starts a new connection, then by definition it can't share the transactional context (presuming no XA or anything as far as I know), so the context is completely different.

I'm struggling to understand how this was a "bug in withNewSession" and not a fundamental change in what and how this method is used.

At a minimum, the documentation needs to change to reflect the change in behavior and possibly give some references to how or why it should be used.