update core_shell_cylinder docs to address #1030
update core_shell_cylinder docs to address #1030

Updated docs to point out the core_shell_cylinder includes end caps.
Point users to core_shell_bicelle if they want a core_shell_cylinder
without end caps (by setting face thickness to zero) -- setting face
thickness to zero was tested and verified that setting that parameter to
zero does not cause a problem
SasView/sasmodels,SasView/sasmodels,SasView/sasmodels,SasView/sasmodels