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 openzfs#12111

man/man8/zfsprops.8

@@ -38,7 +38,7 @@
 .\" Copyright 2019 Joyent, Inc.
 .\" Copyright (c) 2019, Kjeld Schouten-Lebbing
 .\"
-.Dd September 1, 2020
+.Dd May 24, 2021
 .Dt ZFSPROPS 8
 .Os
 .Sh NAME
@@ -1197,15 +1197,10 @@ location.
 .It Sy nbmand Ns = Ns Sy on Ns | Ns Sy off
 Controls whether the file system should be mounted with
 .Sy nbmand
-.Pq Non Blocking mandatory locks .
+.Pq Non-blocking mandatory locks .
 This is used for SMB clients.
 Changes to this property only take effect when the file system is umounted and
-remounted.
-See
-.Xr mount 8
-for more information on
-.Sy nbmand
-mounts. This property is not used on Linux.
+remounted. Support for these locks is scarce and not described by POSIX.
 .It Sy overlay Ns = Ns Sy on Ns | Ns Sy off
 Allow mounting on a busy directory or a directory which already contains
 files or directories.