add option shade to smoothTrend and TheilSen, update documentation

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/openair@838 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
more doc refinements

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/openair@839 edb9625f-4e0d-4859-8d74-9fd3b1da38cb