thermal: sysfs-api.txt - document passive attribute for thermal zones

Signed-off-by: NFrans Pop <elendil@planet.nl>
Acked-by: NZhang Rui <rui.zhang@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLen Brown <len.brown@intel.com>

Documentation/thermal/sysfs-api.txt

@@ -199,6 +199,15 @@ cdev[0-*]_trip_point
 	point.
 	RO, Optional
 
+passive
+	Attribute is only present for zones in which the passive cooling
+	policy is not supported by native thermal driver. Default is zero
+	and can be set to a temperature (in millidegrees) to enable a
+	passive trip point for the zone. Activation is done by polling with
+	an interval of 1 second.
+	Unit: millidegrees Celsius
+	RW, Optional
+
 *****************************
 * Cooling device attributes *
 *****************************