Add a note that rb(3) is preferred for new code.

share/man/man3/tree.3

@@ -1,4 +1,4 @@
-.\"	$NetBSD: tree.3,v 1.5 2009/02/14 22:07:04 dholland Exp $
+.\"	$NetBSD: tree.3,v 1.6 2010/05/05 06:24:43 jruoho Exp $
 .\"	$OpenBSD: tree.3,v 1.9 2003/05/20 09:13:38 jmc Exp $
 .\"/*
 .\" * Copyright 2002 Niels Provos <provos@citi.umich.edu>
@@ -29,7 +29,7 @@
 .\" * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\" */
-.Dd February 14, 2009
+.Dd May 5, 2010
 .Dt TREE 3
 .Os
 .Sh NAME
@@ -131,6 +131,12 @@
 .Ft "struct TYPE *"
 .Fn RB_REMOVE "NAME" "RB_HEAD *head" "struct TYPE *elm"
 .Sh DESCRIPTION
+.Bf -symbolic
+This is a legacy interface; for new code,
+.Xr rb 3
+is preferred.
+.Ef
+.Pp
 These macros define data structures for different types of trees:
 splay trees and red-black trees.
 .Pp