Add polygon orientation to ClipClosedSurface doc

Add comment to explain the importance of front face vs. back face when using a surface to define a solid:

The orientation of the polygons that form the surface is important. Polygons have a front face and a back face, and it's the back face that defines the interior or "solid" region of the closed surface. When a clipping plane cuts through a "solid" region, a new cut face is generated, but not when a clipping plane cuts through a hole or "empty" region. This distinction is crucial when dealing with complex surfaces. Note that if a simple surface has its back faces pointing outwards, then that surface defines a hole in a potentially infinite solid.