The tag is present in Lintian version 2.104.234. That is the most recent version we know about.
If the package installs a symbolic link
/usr/share/doc/pkg1 -> pkg2, then pkg1
and pkg2 must both come from the same source package.
The best solution is probably to stop symlinking the
/usr/share/doc directory for this package and instead include a
real /usr/share/doc/pkg1 directory within pkg1 with the
appropriate contents (such as the
Refer to Debian Policy Manual section 12.5 for details.