Update documentation to clarify that FQDNs configured in DHCP (e.g. next-server) are resolved only once, on startup

It is not clear in the ISC DHCP documentation that when an option that will be used operationally as an IP address is configured using a FQDN, that this will be resolved only once and the resulting IP address used thereafter and never refreshed.

In other words, DHCP is doing a one-time resolution of the FQDN that had been configured for convenience instead of using an address. The TTL of the DNS query response is not observed and the FQDN will not be resolved again until the server is restarted.

This might lead to mis-set expectations when designing a deployment that assumes that it is possible (by means of rotating DNS query responses) to load balance some client provisioning. For example: Support ticket 14783