Update doc to explain the new feature

Signed-off-by: Maxime CLEMENT <maxime.clement@tier4.jp>

Author: maxime-clem

planning/behavior_velocity_dynamic_obstacle_stop_module/README.md

@@ -38,8 +38,10 @@ An object is considered by the module only if it meets all of the following cond
 - it is a vehicle (pedestrians are ignored);
 - it is moving at a velocity higher than defined by the `minimum_object_velocity` parameter;
 - it is not too close to the current position of the ego vehicle;
+- it is not unavoidable (only if parameter `ignore_unavoidance_collisions` is set to `true`);
 - it is close to the ego path.
 
+An object is considered unavoidable if it is heading towards the ego vehicle such that even if ego stops, a collision would still occur (assuming the object keeps driving in a straight line).
 For the last condition,
 the object is considered close enough if its lateral distance from the ego path is less than the threshold parameter `minimum_object_distance_from_ego_path` plus half the width of ego and of the object (including the `extra_object_width` parameter).
 In addition, the value of the `hysteresis` parameter is added to the minimum distance if a stop point was inserted in the previous iteration.