It appears that the mirror options (-N) are not listed for lfs setstripe, only under lfs mirror create. Since these commands are at least sharing the same backend for setstripe, and lfs setstripe is the common way to specify (default) layouts on a directory, it makes sense to include the "-N" option in the lfs-setstripe.1 man page. It can reference lfs-mirror-create.1 for examples.