thermal: re-document thermal units

From: Zhang Rui <rui.zhang@intel.com>

(reverting the previous sysfs patch also reverted a fix
 to the thermal units documentation, which is restored by this commit)

The generic thermal driver shows temperature in millidegree Celsius.

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