Document Corpus alias (to VCorpus)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1370 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Recognize meta argument, check for id and language in readXML


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/tm@1371 edb9625f-4e0d-4859-8d74-9fd3b1da38cb