hwmon: (dell-smm) Improve documentation

Notify users of this driver that each pwmX
attribute controls fan number X, meaning that
probing of pwm channels is unnecessary.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20220727083004.5684-1-W_Armin@gmx.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>