/usr/lib/ghc-doc/hoogle/libghc-hmatrix-gsl-doc.txt is in libghc-hmatrix-gsl-doc 0.18.0.1-1build2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-hmatrix-gsl-doc/html/hmatrix-gsl.txt