+str #24229 implement flattening of mat values, may block; SHOULD BE INTERNAL API

add java documentation

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akka-stream/src/main/scala/akka/stream/javadsl/Sink.scala

@@ -18,7 +18,10 @@ import scala.concurrent.ExecutionContext
 import scala.util.Try
 import java.util.concurrent.CompletionStage
 
+import akka.annotation.InternalApi
+
 import scala.compat.java8.FutureConverters._
+import scala.concurrent.duration.FiniteDuration
 
 /** Java API */
 object Sink {
@@ -333,6 +336,13 @@ final class Sink[-In, +Mat](delegate: scaladsl.Sink[In, Mat]) extends Graph[Sink
   def mapMaterializedValue[Mat2](f: function.Function[Mat, Mat2]): Sink[In, Mat2] =
     new Sink(delegate.mapMaterializedValue(f.apply _))
 
+  /**
+   * INTERNAL API: Unsafe BLOCKING flattening if current materialized value is a Future.
+   */
+  @InternalApi
+  private[akka] def flattenMaterializedValue[Mat2](timeout: FiniteDuration): Sink[In, Mat2] =
+    new Sink(delegate.flattenMaterializedValue[Mat2](timeout))
+
   /**
    * Replace the attributes of this [[Sink]] with the given ones. If this Sink is a composite
    * of multiple graphs, new attributes on the composite will be less specific than attributes