Warning: This page is seriously outdated since it predates dpkg-maintscript-helper symlink_to_dir/dir_to_symlink. Nowadays you would just create debian/mlterm-im-ibus.maintscript:

dir_to_symlink /usr/share/doc/mlterm-im-ibus mlterm-common 3.1.2-1.1~

This page is meant to describe how to address the problem "missing copyright file" as meant here. The cause is that dpkg refuses to replace a directory with a symlink or vice versa (this behaviour is wanted for unrelated reasons).

If you want to replace a directory with a symlink, you may let a preinst script remove the directory before dpkg creates the symbolic link. Example from the mlterm package:

   1 #!/bin/sh
   2 set -e
   4 case "$1" in
   5     install|upgrade)
   6         # dpkg does not replace directories by symlinks or vice versa.
   7         if dpkg --compare-versions "$2" lt "3.1.2-1.1" ; then
   8           rm -rf /usr/share/doc/mlterm-im-ibus || true
   9         fi
  10     ;;
  11     abort-upgrade)
  12     ;;
  13     *)
  14         echo "preinst called with unknown argument \`$1'" >&2
  15         exit 1
  16     ;;
  17 esac
  21 exit 0

The other obvious solution is not to use symlinks at all. Symbolic links created by any previous version must then be removed by a preinst script. In case you did replace a symlink with a directory in version 1, but are adding the preinst script a posteriori in version 3, a system with version 2 may still contain a symlink (because dpkg did ignore your changes in a normal upgrade path) or not (because the user did try version 3 then downgrade). You should then remove any existing symlink, whatever the upgraded version, as done here.