rename arg 'iid' to 'correlated' and document

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2611 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
implement correlated simulation for cca, and clearly skip for capscale

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2612 edb9625f-4e0d-4859-8d74-9fd3b1da38cb