ACPICA: Update comments about behavior when _STA does not exist

ACPICA: Update comments about behavior when _STA does not exist

No functional change.  Add some comments concerning behavior
when the _STA method does not exist. According to the ACPI
specification, in this case the device should be assumed to be
present, functional, and enabled.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>