Improve documentation for the more precise spatial intersection/encapsulation check

[mmeijerdfki]

The images of the description in #417 should be documented in the git repo itself.
Another thing that could be mentioned is:
If add_face of a CGAL::SurfaceMesh is used, often the vertices need to be reversed, or otherwise the face is not valid and the add_face function returns a CGAL::SurfaceMesh::null_face.
This leads to problems when checking the intersection of two SurfaceMeshes.