/usr/lib/ghc-doc/hoogle/hxt-relaxng.txt is in libghc-hxt-relaxng-doc 9.1.4-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-hxt-relaxng-doc/html/hxt-relaxng.txt