Added comments here and in wiki.

      //  - If a link has no sphere_approximation then one will be generated
      //     automatically containing a single sphere which encloses the entire
      //     collision geometry.
      //  - If a link has only spheres with radius 0 then it will not be
      //     considered for collision detection.  In this case the internal
      //     representation is a single radius=0 sphere.
      //  - If a link has at least one sphere with radius>0 then those spheres
      //     are used.  Any radius=0 spheres are eliminated.