ACPI: thinkpad-acpi: update CMOS commands documentation

The CMOS set of commands is often just used to keep the CMOS NVRAM in sync
with whatever the ACPI BIOS has been doing in modern ThinkPads.  In older
ThinkPads, it actually carried out real actions.  Document this.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>