Improve manpage and config file to show the more common use case.

from Nick Holland (nick AT holland-consulting DOT net)
ok jmc@ florian@

src/etc/acme-client.conf

@@ -1,5 +1,5 @@
 #
-# $OpenBSD: acme-client.conf,v 1.3 2017/01/21 09:06:57 benno Exp $
+# $OpenBSD: acme-client.conf,v 1.4 2017/03/22 11:14:14 benno Exp $
 #
 authority letsencrypt {
         agreement url "https://letsencrypt.org/documents/LE-SA-v1.1.1-August-1-2016.pdf"
@@ -17,6 +17,6 @@ authority letsencrypt-staging {
 #	alternative names { secure.example.com }
 #	domain key "/etc/ssl/private/example.com.key"
 #	domain certificate "/etc/ssl/example.com.crt"
-#	domain chain certificate "/etc/ssl/example.com.chain.pem"
+#	domain full chain certificate "/etc/ssl/example.com.fullchain.pem"
 #	sign with letsencrypt
 #}