Improve RETURN VALUES section by mentioning SIG_ERR, per lib/15686, and fix

some spacing.

lib/libc/gen/signal.3

@@ -1,4 +1,4 @@
-.\"	$NetBSD: signal.3,v 1.16 2002/02/07 07:00:17 ross Exp $
+.\"	$NetBSD: signal.3,v 1.17 2002/02/21 11:09:07 wiz Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -59,7 +59,7 @@ facility.
 .Pp
 Signals allow the manipulation of a process from outside its
 domain as well as allowing the process to manipulate itself or
-copies of itself (children). There are two general types of signals:
+copies of itself (children).  There are two general types of signals:
 those that cause termination of a process and those that do not.
 Signals which cause termination of a program might result from
 an irrecoverable error or might be the result of a user at a terminal
@@ -83,7 +83,7 @@ and
 signals, the
 .Fn signal
 function allows for a signal to be caught, to be ignored, or to generate
-an interrupt. See
+an interrupt.  See
 .Xr signal 7
 for comprehensive list of supported signals.
 .Pp
@@ -102,7 +102,7 @@ To ignore the signal
 should be
 .Dv SIG_IGN .
 This will cause subsequent instances of the signal to be ignored
-and pending instances to be discarded. If
+and pending instances to be discarded.  If
 .Dv SIG_IGN
 is not used,
 further occurrences of the signal are
@@ -150,7 +150,9 @@ function;
 ignored signals remain ignored.
 .Sh RETURN VALUES
 The previous action is returned on a successful call.
-Otherwise, \-1 is returned and  the global variable
+Otherwise,
+.Dv SIG_ERR
+is returned and the global variable
 .Va errno
 is set to indicate the error.
 .Sh ERRORS