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

Add documentation for fan_mult and fan_max.

Signed-off-by: Armin Wolf <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			[HW] Allow controlling fans only if SYS_ADMIN
 			capability is set.
 
+	dell_smm_hwmon.fan_mult=
+			[HW] Factor to multiply fan speed with.
+
+	dell_smm_hwmon.fan_max=
+			[HW] Maximum configurable fan speed.
+
 	dfltcc=		[HW,S390]
 			Format: { on | off | def_only | inf_only | always }
 			on:       s390 zlib hardware support for compression on