Replace the nonstandard fanX_source attributes with the standardized
pwmX_auto_channels_temp attributes and document the special behaviour
associated with those attributes.

Tested on a Fujitsu DS3401-B1.
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20230105225107.58308-3-W_Armin@gmx.deSigned-off-by: Guenter Roeck <linux@roeck-us.net>