Add someting about winding order (or lack thereof) to the Tetrahedron::face documentation.

Hello!

I've noticed that the method https://docs.rs/ncollide/0.14.1/x86_64-pc-windows-msvc/ncollide/shape/struct.Tetrahedron.html#method.face does not return triangles in a consistent winding order.

Specifically, faces ABC and ABD have opposite vertex winding order (note how edge AB is included in both in the same order).

Although the method does not claim to provide a consistent winding order, one (read: me) might expect to be able to just call this method and get the normal of the triangle to get the normal of the face.

Perhaps a note about this should be added.