/usr/share/doc/libgfortran-6-dev-armhf-cross is in libgfortran-6-dev-armhf-cross 6.3.0-18cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/gcc-6-cross-base
/usr/share/doc/gcc-6-cross-base