wifi: Update help/notes for forcing regulatory domain

With recent changes the regulatory domain is handled via the hostap
rather than by passing and going through the driver and hostap doesn't
have any option for forcing the regulatory domain, so, add a note to
reflect that this is implementation dependent to cover both hostap as
well other offloaded implementations.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>