/usr/share/doc/gtksourceviewmm-3.0 is in libgtksourceviewmm-3.0-doc 3.2.0-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgtksourceviewmm-3.0-doc
/usr/share/doc/libgtksourceviewmm-3.0-doc