Various documentation improvements

[jerryz123]

1. The rectangles for all objects are in the (centerx, centery, dimx, dimy) format. I was initially trying to work with them in the (topleftx, topleft-y, dimx, dimy) format.
2. From what I could tell, the directionality of the lanes is dictated by the x dimension, this would also save anyone else creating lanes some common mishaps.
3. If the width of the lane is large enough to accommodate two cars, that doesn't mean both cars can generate valid paths. For an example of this, see the attached picture where I think the problem becomes apparent. This is only an issue with background cars, but could still be useful to know.
4. The position of the traffic light is ambiguous. I wasn't sure how close to the street it had to be for it to start working, I just tweaked it until it did. Any documentation here would also be really helpful.