/usr/share/doc/gtkspellmm-3.0 is in libgtkspellmm-3.0-doc 3.0.5+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgtkspellmm-3.0-doc
/usr/share/doc/libgtkspellmm-3.0-doc