thermal: core: Rewrite comments in handle_thermal_trip()

Make the comments regarding trip crossing and threshold updates in
handle_thermal_trip() slightly more clear.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>