Move the private header to public.
Make documentation a part of public interface.
[ChangeLog][QtCore][QAtomicScopedValueRollback] New class.
Task-number: QTBUG-115107
Change-Id: I6c9f5448e74a5b62f4d97ee079944f4b1b731121
Reviewed-by: Marc Mutz <marc.mutz@qt.io>