Note explicitly that memcpy does not guarantee good behavior on overlap.

lib/libc/string/memcpy.3

@@ -34,7 +34,7 @@
 .\" SUCH DAMAGE.
 .\"
 .\"     from: @(#)memcpy.3	8.1 (Berkeley) 6/4/93
-.\"	$NetBSD: memcpy.3,v 1.7 2001/07/06 15:11:18 tv Exp $
+.\"	$NetBSD: memcpy.3,v 1.8 2001/07/07 04:53:52 perry Exp $
 .\"
 .Dd June 4, 1993
 .Dt MEMCPY 3
@@ -58,6 +58,9 @@ bytes from string
 .Fa src
 to string
 .Fa dst .
+The arguments must not overlap -- behavior if the arguments overlap is
+undefined. To copy byte strings that overlap, use
+.Xr memmove 3 .
 .Sh RETURN VALUES
 The
 .Fn memcpy