The tag is present in Lintian version 2.108.13. That is the most recent version we know about.
The strings "GNU Free Documentation License" or "GFDL" appear in the
copyright file for this package, but the copyright file does not
/usr/share/common-licenses as the location of the GFDL
on Debian systems.
If the copyright file must mention the GFDL for reasons other than stating the license of the package, please add a Lintian override.
Please refer to Debian Policy Manual section 12.5 for details.