Clarify that strerror() doesn't return EINVAL but stores in errno;

prompted by Thomas Klausner.

lib/libc/string/strerror.3

@@ -1,4 +1,4 @@
-.\" $NetBSD: strerror.3,v 1.12 2005/07/29 13:14:49 wiz Exp $
+.\" $NetBSD: strerror.3,v 1.13 2006/05/22 21:40:33 kleink Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -32,7 +32,7 @@
 .\" SUCH DAMAGE.
 .\"
 .\"     @(#)strerror.3	8.1 (Berkeley) 6/9/93
-.Dd July 28, 2005
+.Dd May 22, 2006
 .Dt STRERROR 3
 .Os
 .Sh NAME
@@ -99,17 +99,20 @@ a colon and space
 .Pq Dq Li ":\ " ;
 otherwise, only the error message string is printed.
 .Pp
-If the error number is not recognized, these functions return an error message
+If the error number is not recognized, these functions pass an error message
 string containing
 .Dq Li "Unknown error:\ "
 followed by the error number in decimal.
-The
+To warn about this,
 .Fn strerror
+sets
+.Dv errno
+to
+.Er EINVAL,
 and
 .Fn strerror_r
-functions return
-.Er EINVAL
-as a warning.
+returns
+.Er EINVAL .
 Error numbers recognized by this implementation fall in
 the range 0 \*[Lt]
 .Fa errnum