Comment 12 for bug 303578

dpkg wont convert a directory to a symlink or vice-versa on upgrades; that is if an older version of ubuntu-docs used to ship a /foo/bar symlink and the new version ships a /foo/bar dir, it will result in a symlink. (This is considered a feature of dpkg which allows symlinking e.g. /usr to /mnt/large-space-device).

So I'd look at the .deb history and see whether it used to ship a directory in place of the symlink and fix this in the preinst.