hwmon: (jc42) more helpful documentation

The documentation lists standard numbers and chip names in excruciating
detail, but that's all it does.  To help mere mortals in deciding
whether to enable this driver, mention what this sensor is for and in
which systems it might be found.

Also add a link to the actual JC 42.4 specification.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: stable@kernel.org
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>