more docs for Sink.queue and actorRefWithAck (#25063)

akka-docs/src/main/paradox/stream/operators/Source/queue.md

@@ -1,4 +1,4 @@
-# queue
+# Source.queue
 
 Materialize a `SourceQueue` onto which elements can be pushed for emitting from the source. 
 
@@ -19,6 +19,21 @@ a buffer, if elements are pushed onto the queue faster than the source is consum
 a strategy specified by the user. Functionality for tracking when an element has been emitted is available through
 `SourceQueue.offer`.
 
+Using `Source.queue` you can push elements to the queue and they will be emitted to the stream if there is
+demand from downstream, otherwise they will be buffered until request for demand is received. Elements in the buffer
+will be discarded if downstream is terminated.
+
+In combination with the queue, the @ref[`throttle`](./../Source-or-Flow/throttle.md) operator can be used to control the processing to a given limit, e.g. `5 elements` per `3 seconds`.
+
+## Example
+
+Scala
+:   @@snip [IntegrationDocSpec.scala]($code$/scala/docs/stream/IntegrationDocSpec.scala) { #source-queue }
+
+Java
+:   @@snip [IntegrationDocTest.java]($code$/java/jdocs/stream/IntegrationDocTest.java) { #source-queue }
+
+## Reactive Streams Semantics
 
 @@@div { .callout }