ACPI: update comments for acpi_event_status

ACPI_EVENT_FLAG_HANDLE is a flag for acpi_event_status.
When it is set, it indicates that the ACPI event,
either GPE or fixed event, is associated with a handler.

Update the comments to reflect this flag.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>