Clarify which IP is meant

[enygren]

In 6.3 we say: "When performing discovery using resolver IP addresses,
clients MUST use the IP address as the URI host for DoH requests."

There are multiple IP addresses that may be in-play here. We should clarify
which is "the IP address" in the case where the Designated Resolver and
the Unencrypted Resolver do NOT share an IP. (I take it that this is the IP
of the Unencrypted Resolver ?)

[tfpauly]

Good point!