/usr/lib/ghc-doc/hoogle/mtl.txt is in libghc-mtl-doc 2.1.2-4.
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