make it crystal clear that errstr is NULL after success

src/lib/libc/stdlib/strtonum.3

@@ -1,4 +1,4 @@
-.\" $OpenBSD: strtonum.3,v 1.9 2005/02/07 08:54:59 jmc Exp $
+.\" $OpenBSD: strtonum.3,v 1.10 2005/04/14 02:13:29 tedu Exp $
 .\"
 .\" Copyright (c) 2004 Ted Unangst
 .\"
@@ -80,6 +80,9 @@ unless the value would exceed the provided bounds or is invalid.
 On error, 0 is returned and
 .Fa errstr
 will point to an error message.
+.Fa *errstr
+will be set to NULL on success; this fact can be used to differentiate
+a successful return of 0 from an error.
 .Sh EXAMPLES
 Using
 .Fn strtonum