Add a bold note to say our PF is obsolete.

dist/pf/share/man/man4/pf.4

@@ -1,4 +1,4 @@
-.\"	$NetBSD: pf.4,v 1.12 2009/12/19 14:05:53 ahoka Exp $
+.\"	$NetBSD: pf.4,v 1.13 2018/08/01 13:30:13 maxv Exp $
 .\"	$OpenBSD: pf.4,v 1.59 2007/05/31 19:19:51 jmc Exp $
 .\"
 .\" Copyright (C) 2001, Kjell Wooding.  All rights reserved.
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd December 19, 2009
+.Dd August 1, 2018
 .Dt PF 4
 .Os
 .Sh NAME
@@ -36,6 +36,13 @@
 .Sh SYNOPSIS
 .Cd "pseudo-device pf"
 .Sh DESCRIPTION
+.Bf -symbolic
+The NetBSD version of PF is obsolete, and its use is strongly discouraged.
+Use
+.Xr npf 7
+instead.
+.Pp
+.Ef
 Packet filtering takes place in the kernel.
 A pseudo-device,
 .Pa /dev/pf ,