Thermal: Documentation update

With commit 6503e5df,
the value of /sys/class/thermal/thermal_zoneX/mode has been changed
from user/kernel to enabled/disabled.
Update the documentation so that users won't be confused.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>