cu.1: describe better + tag spdx

Preserve over 40 years of "call UNIX" BSD heritage
while answering "one line about what it does" e.g.

"how do I get a serial console?"
"% apropos serial"

MFC after:	3 days
Reported by:	imp

usr.bin/tip/tip/cu.1

@@ -1,3 +1,6 @@
+.\"-
+.\" SPDX-License-Identifier: BSD-3-Clause
+.\"
 .\" $OpenBSD: cu.1,v 1.3 2006/06/07 06:35:59 mbalmer Exp $
 .\"
 .\" Copyright (c) 1980, 1990, 1993
@@ -32,7 +35,7 @@
 .Os
 .Sh NAME
 .Nm cu
-.Nd call UNIX
+.Nd call UNIX over a serial line
 .Sh SYNOPSIS
 .Nm
 .Op Fl ehot