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

Add details of the interface devm_thermal_zone_of_sensor_register()
and devm_thermal_zone_of_sensor_unregister() in the
<thermal/sysfs-api.txt>.

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