Documentation: admin-guide: Add Documentation for undocumented dell_smm_hwmon parameters

Add documentation for fan_mult and fan_max.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20220109214248.61759-3-W_Armin@gmx.de


Signed-off-by: Guenter Roeck <linux@roeck-us.net>