thermal: add description for integral_cutoff unit

Add more explicitly description for unit of integral_cutoff which used
by power allocator governor.
Signed-off-by: NLeo Yan <leo.yan@linaro.org>
Acked-by: NJavi Merino <javi.merino@arm.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/thermal/sysfs-api.txt

@@ -364,6 +364,7 @@ integral_cutoff
 	accumulates error when temperature is above the desired
 	temperature trip point. For more information see
 	Documentation/thermal/power_allocator.txt
+	Unit: millidegree Celsius
 	RW, Optional
 
 slope