/usr/share/gtk-doc/html/gio is in libglib2.0-doc 2.56.2-0ubuntu0.18.04.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/libglib2.0-doc/gio
/usr/share/gtk-doc/html/../../doc/libglib2.0-doc/gio