small edits to help pages

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@33 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
adjusted naming of node grobs

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@34 edb9625f-4e0d-4859-8d74-9fd3b1da38cb