/usr/lib/ghc-doc/hoogle/stm.txt is in libghc-stm-doc 2.4.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-stm-doc/html/stm.txt
/usr/lib/ghc-doc/hoogle/../../../share/doc/libghc-stm-doc/html/stm.txt