+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/Source.scala

@@ -25,6 +25,7 @@ import scala.compat.java8.OptionConverters._
 import java.util.concurrent.CompletionStage
 import java.util.concurrent.CompletableFuture
 
+import akka.annotation.InternalApi
 import akka.stream.scaladsl.Sink
 
 import scala.compat.java8.FutureConverters._
@@ -484,6 +485,13 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   def mapMaterializedValue[Mat2](f: function.Function[Mat, Mat2]): Source[Out, Mat2] =
     new Source(delegate.mapMaterializedValue(f.apply _))
 
+  /**
+   * INTERNAL API: Unsafe BLOCKING flattening if current materialized value is a Future.
+   */
+  @InternalApi
+  private[akka] def flattenMaterializedValue[Mat2](timeout: FiniteDuration): Source[Out, Mat2] =
+    new Source(delegate.flattenMaterializedValue[Mat2](timeout))
+
   /**
    * Transform this [[Source]] by appending the given processing stages.
    * {{{