/usr/lib/ghc-doc/hoogle/libghc-mtl-doc.txt is in libghc-mtl-doc 2.2.1-5build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-mtl-doc/html/mtl.txt
/usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-mtl-doc/html/mtl.txt