/usr/lib/ghc-doc/hoogle/utf8-string.txt is in libghc-utf8-string-doc 0.3.7-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-utf8-string-doc/html/utf8-string.txt