drivers/acpi: Document why AcpiOsUnmapMemory() is a no-op

Add the explanation by Avi Kivity to AcpiOsUnmapMemory() to explain why
it's a no-op.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>