#11 add comments for subtle feature in BoxObstacle

[Craig Reynolds]

see the reference to BoxObstacle::findIntersectionWithVehiclePath in https://sourceforge.net/forum/message.php?msg_id=4116493

The steerHint for BoxObstacle is significantly different than for a collection of six RectangleObstacles but I had to look at the code twice to find the relevant bit. It should be highlighted with a comment