more vocal about using adjusted R2 in varpart + edits

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2592 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
remove a case of stats:::

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2593 edb9625f-4e0d-4859-8d74-9fd3b1da38cb