document that --version prints to stderr

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2642 25d90789-57f7-4ee0-8453-03a3dfeeeb22
sync with wiki

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2643 25d90789-57f7-4ee0-8453-03a3dfeeeb22