/usr/share/doc/gcc-8-multilib-mips64-linux-gnuabi64 is in gcc-8-multilib-mips64-linux-gnuabi64 8-20180414-1ubuntu2cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-8-mips64-linux-gnuabi64-base