Expand the selectpoints3d.Rd example.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1016 79a1b648-8b68-254f-b20b-c0b50f5137bf
typo

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1017 79a1b648-8b68-254f-b20b-c0b50f5137bf
background could be NULL

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rgl@1018 79a1b648-8b68-254f-b20b-c0b50f5137bf