/usr/bin/texdoctk is in texlive-base 2015.20160320-1ubuntu0.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/bin/../share/texlive/texmf-dist/scripts/texdoctk/texdoctk.pl
/usr/bin/../share/texlive/texmf-dist/scripts/texdoctk/texdoctk.pl