hwmon: Improve the pwmN_enable documentation

The documentation of the pwmN_enable interface file is not very clear,
and has been confusing several driver authors already. Make it clearer.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NMark M. Hoffman <mhoffman@lightlink.com>

Documentation/hwmon/sysfs-interface

@@ -172,11 +172,10 @@ pwm[1-*]	Pulse width modulation fan control.
 		255 is max or 100%.
 
 pwm[1-*]_enable
-		Switch PWM on and off.
-		Not always present even if pwmN is.
-		0: turn off
-		1: turn on in manual mode
-		2+: turn on in automatic mode
+		Fan speed control method:
+		0: no fan speed control (i.e. fan at full speed)
+		1: manual fan speed control enabled (using pwm[1-*])
+		2+: automatic fan speed control enabled
 		Check individual chip documentation files for automatic mode
 		details.
 		RW