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

Documentation/hwmon/dell-smm-hwmon.rst

@@ -46,6 +46,9 @@ temp[1-10]_input                RO      Temperature reading in milli-degrees
 temp[1-10]_label                RO      Temperature sensor label.
 =============================== ======= =======================================
 
+Due to the nature of the SMM interface, each pwmX attribute controls
+fan number X.
+
 Disabling automatic BIOS fan control
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