Documentation: layman friendly terminology in cast_ray max_toi

[andrewbaxter]

In https://rapier.rs/docs/user_guides/rust/scene_queries it says the limit is [ray.origin, ray.origin + ray.dir * max_toi] suggesting that max_toi is in unit of ray.dir (i.e. toi 1 == ray.origin + ray.dir).

The code docstring says limits the length of the ray to ray.dir.norm() * max_toi (i.e. toi 1 could be before or after ray.origin + ray.dir).