Added docstring comments for several ExtrusionEntityCollection methods.
Added docstring comments for several ExtrusionEntityCollection methods.

alexrj/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,alexrj/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,curieos/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r