/usr/share/doc/cinnamon/cinnamon is in cinnamon-doc 3.6.7-8ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/cinnamon/../../gtk-doc/html/cinnamon
/usr/share/doc/cinnamon/../../gtk-doc/html/cinnamon