hwmon: Add description of the update_rate sysfs attribute

The update_rate attribute can be used by drivers to let userspace choose
the update rate of the chip, if it is configurable.

Signed-off-by: Ira W. Snyder <[e-mail]>
Signed-off-by: Jean Delvare <[e-mail]>

Documentation/hwmon/sysfs-interface

···
 given driver if the chip has the feature.
 
 
-********
-* Name *
-********
+*********************
+* Global attributes *
+*********************
 
 name		The chip name.
 		This should be a short, lowercase string, not containing
···
 		the only mandatory attribute.
 		I2C devices get this attribute created automatically.
 		RO
+
+update_rate	The rate at which the chip will update readings.
+		Unit: millisecond
+		RW
+		Some devices have a variable update rate. This attribute
+		can be used to change the update rate to the desired
+		frequency.
 
 
 ************