Add note about "netstat -mssv".

usr.bin/netstat/netstat.1

@@ -1,4 +1,4 @@
-.\"	$NetBSD: netstat.1,v 1.75 2022/01/07 10:17:39 bad Exp $
+.\"	$NetBSD: netstat.1,v 1.76 2022/12/21 09:06:59 msaitoh Exp $
 .\"
 .\" Copyright (c) 1983, 1990, 1992, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"	@(#)netstat.1	8.8 (Berkeley) 4/18/94
 .\"
-.Dd July 21, 2020
+.Dd December 21, 2022
 .Dt NETSTAT 1
 .Os
 .Sh NAME
@@ -239,6 +239,12 @@ is used.
 .It Fl m
 Show statistics recorded by the mbuf memory management routines
 (the network manages a private pool of memory buffers).
+If the kernel option
+.Cd options MBUFTRACE
+is set, extra info can be retrieved with
+.Fl mssv .
+See also
+.Xr options 4 .
 .It Fl N Ar system
 Use
 .Xr kvm 3
@@ -436,6 +442,7 @@ And the fifth character shows the header complete flag status.
 .Xr kvm_openfiles 3 ,
 .Xr sysctl 3 ,
 .Xr bpf 4 ,
+.Xr options 4 ,
 .Xr route 4 ,
 .Xr hosts 5 ,
 .Xr networks 5 ,