hwmon: (adm1021) Clarify documentation regarding Xeon processors

Recent Xeon processor thermal sensors are supported by the coretemp
driver and not the adm1021 driver. Only one old generation of Xeon
processors (the first Netburst ones) are supported by the adm1021
driver.

Reported-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>