/usr/lib/ghc-doc/hoogle/fingertree.txt is in libghc-fingertree-doc 0.1.0.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-fingertree-doc/html/fingertree.txt