doc: clarify setServers() methods in dns.md

Added a note that that clarifies the fact that setServers() does not
check subsequent servers when the first one produces a NOTFOUND error.

PR-URL: https://github.com/nodejs/node/pull/21469
Refs: https://github.com/nodejs/node/issues/21391


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>