minor improvement in the documentation of selection()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1504 d36ae590-6c38-44c5-9ba5-7217e93607d8
improved and extended test script for binary selection models

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sampleselection@1505 d36ae590-6c38-44c5-9ba5-7217e93607d8