/usr/lib/ghc-doc/hoogle/libghc-mbox-doc.txt is in libghc-mbox-doc 0.3.1-3build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-mbox-doc/html/mbox.txt