The tag is present in Lintian version 2.104.328. That is the most recent version we know about.
This package contains a *.devhelp or *.devhelp2 file which is not in
the devhelp search path (
/usr/share/gtk-doc/html) and is apparently not in a directory
linked into the devhelp search path. This will prevent devhelp from
finding the documentation.
If the devhelp documentation is installed in a path outside the devhelp
search path (such as
/usr/share/doc), create a symlink in
/usr/share/gtk-doc/html pointing to the documentation directory.