[#28] Clarify that domain names are only resolved once

common/dhcp-options.5 - added text to ip-address value section.

common/dhcp-options.5

 .\"	$Id: dhcp-options.5,v 1.50 2011/05/20 13:48:32 tomasz Exp $
 .\"
-.\" Copyright (c) 2004-2018 by Internet Systems Consortium, Inc. ("ISC")
+.\" Copyright (c) 2004-2019 by Internet Systems Consortium, Inc. ("ISC")
 .\" Copyright (c) 1996-2003 by Internet Software Consortium
 .\"
 .\" Permission to use, copy, modify, and distribute this software for any
@@ -53,7 +53,14 @@ The
 data type can be entered either as an explicit IP
 address (e.g., 239.254.197.10) or as a domain name (e.g.,
 haagen.isc.org).  When entering a domain name, be sure that that
-domain name resolves to a single IP address.
+domain name resolves to a single IP address. Additionally, please
+note the software (dhcpd or dhclient) will only attempt to resolve
+the domain name the first time the option is needed. For example, if
+the next-server option is defined as a domain name, dhcpd will
+attempt to resolve it while responding to the first client query
+dhcpd receives after startup. Should the domain's address subsequently
+change, the software has to be restarted in order to pick up the
+change.
 .PP
 The
 .B ip6-address