hwmon: (drivetemp) Add usage not describing impact on drive spin-down

On some drives, reading the drive temperature resets the drive's
spin-down timer. If the drive temperature is read too often, affected
drives will never spin down. Add this information as usage note to
the driver documentation.

Reported-by: Peter Sulyok <peter@sulyok.net>
Cc: Peter Sulyok <peter@sulyok.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>