Add an exmaple of unmounting all file systems apart from NFS and MFS

e.g,

	umount -a -t nonfs,mfs

This makes it clear that the "no prefix" only needs to be on the first
file-system argument to -t. The mount(8) man page has a clear example
of this but for some reason umount(8) was missing one.

sbin/umount/umount.8

@@ -1,4 +1,4 @@
-.\"	$NetBSD: umount.8,v 1.14 2005/12/30 10:29:56 mjl Exp $
+.\"	$NetBSD: umount.8,v 1.15 2009/05/17 14:09:15 mjf Exp $
 .\"
 .\" Copyright (c) 1980, 1989, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)umount.8	8.2 (Berkeley) 5/8/95
 .\"
-.Dd April 21, 2004
+.Dd May 17, 2009
 .Dt UMOUNT 8
 .Os
 .Sh NAME
@@ -124,6 +124,17 @@ umount -a -t nfs,mfs
 unmounts all filesystems of the type
 .Tn NFS
 and
+.Tn MFS ,
+whereas the
+.Nm
+command:
+.Bd -literal -offset indent
+umount -a -t nonfs,mfs
+.Ed
+.Pp
+unmounts all file systems except those of type
+.Tn NFS
+and
 .Tn MFS .
 .It Fl v
 Verbose, additional information is printed out as each filesystem