/usr/share/doc/gfortran-6-multilib-mips64el-linux-gnuabi64 is in gfortran-6-multilib-mips64el-linux-gnuabi64 6.3.0-18cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-6-mips64el-linux-gnuabi64-base