zfsprops.8: remove nbmand-not-used-on-Linux and pointer to mount(8)

Linux man-pages' mount(8) points at fcntl(2), as does mount(2),
and support for it is little-used, deprecated, and configurable
since 4.5.

As far as I can tell, FreeBSD doesn't support nbmand at all ‒
mandatory locks are mostly dead

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12111