ACPI: ibm-acpi: document fan control

This patch documents the ThinkPad fan control strategies.  Source of the
data:

0. ibm-acpi source
1. DSDTs for various ThinkPads (770, X31, X40, X41, T43, A21m, T22)
2. http://thinkwiki.org/wiki/Embedded_Controller_Firmware#Firmware_Issues
3. http://thinkwiki.org/wiki/How_to_control_fan_speed


4. Various threads about windows fan control utilities in thinkpads.com

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>