Use MAP_FAILED in the return values description.

lib/libc/sys/mmap.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: mmap.2,v 1.14 1998/03/28 16:58:58 kleink Exp $
+.\"	$NetBSD: mmap.2,v 1.15 1998/06/03 22:03:44 kleink Exp $
 .\"
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -163,9 +163,19 @@ should be done.
 Upon successful completion,
 .Nm mmap
 returns a pointer to the mapped region.
-Otherwise, a value of -1 is returned and
+Otherwise, a value of
+.Dv MAP_FAILED
+is returned and
 .Va errno
 is set to indicate the error.
+The symbol
+.Dv MAP_FAILED
+is defined in the header
+.Ao Pa sys/mman.h Ac .
+No successful return from
+.Fn mmap
+will return the value
+.Dv MAP_FAILED .
 .Sh ERRORS
 .Fn Mmap
 will fail if: