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: #21469
Refs: #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>

doc/api/dns.md

@@ -568,6 +568,13 @@ An error will be thrown if an invalid address is provided.
 The `dns.setServers()` method must not be called while a DNS query is in
 progress.
 
+Note that this method works much like
+[resolve.conf](http://man7.org/linux/man-pages/man5/resolv.conf.5.html).
+That is, if attempting to resolve with the first server provided results in a
+`NOTFOUND` error, the `resolve()` method will *not* attempt to resolve with
+subsequent servers provided. Fallback DNS servers will only be used if the
+earlier ones time out or result in some other error.
+
 ## DNS Promises API
 
 > Stability: 1 - Experimental
@@ -1010,6 +1017,13 @@ An error will be thrown if an invalid address is provided.
 The `dnsPromises.setServers()` method must not be called while a DNS query is in
 progress.
 
+Note that this method works much like
+[resolve.conf](http://man7.org/linux/man-pages/man5/resolv.conf.5.html).
+That is, if attempting to resolve with the first server provided results in a
+`NOTFOUND` error, the `resolve()` method will *not* attempt to resolve with
+subsequent servers provided. Fallback DNS servers will only be used if the
+earlier ones time out or result in some other error.
+
 ## Error codes
 
 Each DNS query can return one of the following error codes: