thermal: doc: Add details of thermal_zone_of_sensor_{register,unregister}

Add details of the interface thermal_zone_of_sensor_register() and
thermal_zone_of_sensor_unregister() in the thermal/sysfs-api.txt.

The details describes the functionality and parameter which
are passed to these interfaces.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>