Try to clarify that phi1,phi2 express the ends [phi1,phi2] rather than the start and delta [phi1,phi1+phi2] when creating a tube segement


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@49258 27541ba8-7e3a-0410-8455-c3a389f83636