Cross-references without a web location

Hello,

Is there a way to avoid a non-functional link in the HTML output when
there is no htmlxref.cnf entry for an xref?

Working on the GNU Guix manual, I've noticed that there are several
xrefs for which there is no entry in htmlxref.cnf (because there is no
published web location for them).  In the HTML output, there is still a
link, but it redirects to a nonexistent relative url.  For example, a
reference to the manual "gpm" yields a relative link to "gpm.html",
which of course doesn't work because we don't publish that manual.

It would be nice to instead print something closer to what @inforef
would print, but with the manual title as well.

Thanks,
--
Sarah