Indeed the manpage is referring to the libidn info manual, which is in
the libidn11-dev package. One solution would be to move the info manual
to either the 'idn' package or a separate 'libidn-doc' package. There
is more documentation that we could ship (like the API manual) so maybe
libidn-doc
Verified that the problem also exists in 1.23-2 (Ubuntu 12.04 live
beta).
There doesn't actually appear to be an info page in the package. I
suppose it ought to be included, unless its license is problematic or
something (in which case the fix would be to remove the pointer to the
documentation
For what it's worth, it's probably referring to the libidn info page,
which is in package libidn11-dev. It has a section about the command-
line idn tool and its invocation options.
IMHO, the easiest fix would be to extract the info documentation into a
manual page and remove or reword the SEE
