doc: clarify setServer method

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

Author: Shivang44

doc/api/dns.md

@@ -1010,6 +1010,8 @@ 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 in Linux](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: