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: NIra W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: NJean Delvare <khali@linux-fr.org>

Documentation/hwmon/sysfs-interface

@@ -80,9 +80,9 @@ All entries (except name) are optional, and should only be created in a
 given driver if the chip has the feature.
 
 
-********
-* Name *
-********
+*********************
+* Global attributes *
+*********************
 
 name		The chip name.
 		This should be a short, lowercase string, not containing
@@ -91,6 +91,13 @@ name		The chip name.
 		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.
+
 
 ************
 * Voltages *