Update SYNOPSIS to reflect that y and n options are mutually exclusive.

Also, mention what does biff(1) do if no arguments are given.
Add xref to from(1)

ok wiz@

usr.bin/biff/biff.1

@@ -1,4 +1,4 @@
-.\"	$NetBSD: biff.1,v 1.10 2013/04/28 12:43:51 wiz Exp $
+.\"	$NetBSD: biff.1,v 1.11 2016/12/20 08:21:32 abhinav Exp $
 .\"
 .\" Copyright (c) 1980, 1990, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)biff.1	8.1 (Berkeley) 6/6/93
 .\"
-.Dd April 28, 2013
+.Dd December 20, 2016
 .Dt BIFF 1
 .Os
 .Sh NAME
@@ -37,7 +37,7 @@
 .Nd be notified if mail arrives and who it is from
 .Sh SYNOPSIS
 .Nm
-.Op Cm ny
+.Op Cm n | y
 .Sh DESCRIPTION
 .Nm
 informs the system whether you want to be notified when mail arrives
@@ -62,6 +62,10 @@ or
 .Pa \&.profile
 to be executed at each login.
 .Pp
+When no arguments are present,
+.Nm
+displays the current notification status to the standard output.
+.Pp
 .Nm
 operates asynchronously.
 For synchronous notification use the
@@ -86,6 +90,7 @@ An error occurred.
 .El
 .Sh SEE ALSO
 .Xr csh 1 ,
+.Xr from 1 ,
 .Xr mail 1 ,
 .Xr sh 1 ,
 .Xr comsat 8