Warning for truncated DNS messages (#25666)

* Log warning if DNS response truncated
* Docs msg

akka-actor/src/main/scala/akka/io/dns/internal/DnsClient.scala

@@ -103,6 +103,10 @@ import scala.util.Try
       log.debug(s"Received message from [{}]: [{}]", remote, data)
       val msg = Message.parse(data)
       log.debug(s"Decoded: $msg")
+      // TODO remove me when #25460 is implemented
+      if (msg.flags.isTruncated) {
+        log.warning("DNS response truncated and fallback to TCP is not yet implemented. See #25460")
+      }
       val (recs, additionalRecs) = if (msg.flags.responseCode == ResponseCode.SUCCESS) (msg.answerRecs, msg.additionalRecs) else (Nil, Nil)
       val response = Answer(msg.id, recs, additionalRecs)
       inflightRequests.get(response.id) match {

akka-docs/src/main/paradox/io-dns.md

@@ -1,5 +1,11 @@
 # DNS Extension
 
+@@@ warning
+
+`async-dns` does not currently support TCP fallback and so DNS repsonses may be truncated. See [#25460](https://github.com/akka/akka/issues/25460)
+
+@@@
+
 @@@ note
 
 The `async-dns` API is marked as `ApiMayChange` as more information is expected to be added to the protocol.