docs(behavior_path_planner): fix object_check_min_road_shoulder_width explanation (autowarefoundation#4834)

Signed-off-by: Fumiya Watanabe <rej55.g@gmail.com>

Author: rej55

planning/behavior_path_planner/docs/behavior_path_planner_avoidance_design.md

@@ -192,7 +192,7 @@ The avoidance target should be limited to stationary objects (you should not avo
 Not only the length from the centerline, but also the length from the road shoulder is calculated and used for the filtering process. It calculates the ratio of _the actual length between the the object's center and the center line_ `shift_length` and _the maximum length the object can shift_ `shiftable_length`.
 
 $$
-l_D = l_a - \frac{width}{2} \\
+l_D = l_a - \frac{width}{2}, \\
 ratio =  \frac{l_d}{l_D}
 $$
 
@@ -201,7 +201,7 @@ $$
 - $l_a$ : distance between centerline and most left boundary.
 - $width$ : object width
 
-The closer the object is to the shoulder, the larger the value of $ratio$ (theoretical max value is 1.0), and it compares the value and `object_check_shiftable_ratio` to determine whether the object is a parked-car.
+The closer the object is to the shoulder, the larger the value of $ratio$ (theoretical max value is 1.0), and it compares the value and `object_check_shiftable_ratio` to determine whether the object is a parked-car. If the road has no road shoulders, it uses `object_check_min_road_shoulder_width` as a road shoulder width virtually.
 
 ![fig2](../image/avoidance/parked-car-detection.svg)
 
@@ -692,7 +692,7 @@ namespace: `avoidance.target_filtering.`
 | object_check_backward_distance                        | [m]  | double | Backward distance to search the avoidance target.                                                                                                                                                                                      | 2.0           |
 | object_check_goal_distance                            | [m]  | double | Backward distance to search the avoidance target.                                                                                                                                                                                      | 20.0          |
 | object_check_shiftable_ratio                          | [m]  | double | Vehicles around the center line within this distance will be excluded from avoidance target.                                                                                                                                           | 0.6           |
-| object_check_min_road_shoulder_width                  | [m]  | double | Vehicles around the center line within this distance will be excluded from avoidance target.                                                                                                                                           | 0.5           |
+| object_check_min_road_shoulder_width                  | [m]  | double | Width considered as a road shoulder if the lane does not have a road shoulder target.                                                                                                                                                  | 0.5           |
 | object_last_seen_threshold                            | [s]  | double | For the compensation of the detection lost. The object is registered once it is observed as an avoidance target. When the detection loses, the timer will start and the object will be un-registered when the time exceeds this limit. | 2.0           |
 
 ### Safety check parameters

planning/behavior_path_planner/docs/behavior_path_planner_lane_change_design.md

@@ -163,7 +163,7 @@ First, we divide the target objects into obstacles in the target lane, obstacles
 
 ![object lanes](../image/lane_change/lane_objects.drawio.svg)
 
-Furthermore, to change lanes behind a vehicle waiting at a traffic light, we skip the safety check for the stopping vehicles near the traffic light.
+Furthermore, to change lanes behind a vehicle waiting at a traffic light, we skip the safety check for the stopping vehicles near the traffic light.　The explanation for parked car detection is written in [documentation for avoidance module](../docs/behavior_path_planner_avoidance_design.md).
 
 ##### Collision check in prepare phase
 
@@ -276,7 +276,7 @@ The following parameters are configurable in `lane_change.param.yaml`.
 | `prediction_time_resolution`                | [s]    | double  | Time resolution for object's path interpolation and collision check.                                            | 0.5                |
 | `longitudinal_acceleration_sampling_num`    | [-]    | int     | Number of possible lane-changing trajectories that are being influenced by longitudinal acceleration            | 5                  |
 | `lateral_acceleration_sampling_num`         | [-]    | int     | Number of possible lane-changing trajectories that are being influenced by lateral acceleration                 | 3                  |
-| `object_check_min_road_shoulder_width`      | [m]    | double  | Vehicles around the center line within this distance will be excluded from parking objects                      | 0.5                |
+| `object_check_min_road_shoulder_width`      | [m]    | double  | Width considered as a road shoulder if the lane does not have a road shoulder                                   | 0.5                |
 | `object_shiftable_ratio_threshold`          | [-]    | double  | Vehicles around the center line within this distance ratio will be excluded from parking objects                | 0.6                |
 | `min_length_for_turn_signal_activation`     | [m]    | double  | Turn signal will be activated if the ego vehicle approaches to this length from minimum lane change length      | 10.0               |
 | `length_ratio_for_turn_signal_deactivation` | [-]    | double  | Turn signal will be deactivated if the ego vehicle approaches to this length ratio for lane change finish point | 0.8                |