The "/usr/lib/modules-load.d/" path isn't documented because it's changed to "/lib/modules-load.d/" by
find debian/install/deb/usr/share/man/ -type f | xargs sed -ri 's_/usr(/lib/systemd|/lib/udev|/lib/modules-load.d|/lib/modules\\-load|/lib[^/]|/lib/[^a-z])_\1_g' in systemd's "debian/rules". Since Debian is accepting that packages drop files into "/usr/lib/modules-load.d/", shouldn't the modules-load.d man page list both "/lib/..." and "/usr/lib/..."?