/usr/share/doc/lxpanel-plugin-topmenu/changelog.Debian.gz is in lxpanel-plugin-topmenu 0.2.1+git20151210.8c6108f-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/lxpanel-plugin-topmenu/../libtopmenu-client-gtk2-0/changelog.Debian.gz