Document asSubscriber stage (#28128)

* Unfortunately it seems the jdk9-only tests could not actually be compiled.
With these changes those can actually be compiled and ran again.

* Always link to jdk11 for java.* javadocs

* Update sbt-paradox-akka to fix linking to inner classes for javadoc

akka-docs/src/test/scala-jdk9-only/docs/stream/operators/source/AsSubscriber.scala

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package docs.stream.operators.source;
+
+//#imports
+import java.util.concurrent.Flow.Subscriber;
+import java.util.concurrent.Flow.Publisher;
+
+import akka.NotUsed;
+import akka.stream.scaladsl.Source;
+import akka.stream.scaladsl.JavaFlowSupport;
+
+//#imports
+
+object AsSubscriber {
+    case class Row(name: String)
+
+    class DatabaseClient {
+        def fetchRows(): Publisher[Row] = ???
+    }
+
+    val databaseClient: DatabaseClient = ???;
+
+    // #example
+    val rowSource: Source[Row, NotUsed] =
+      JavaFlowSupport.Source.asSubscriber
+        .mapMaterializedValue(
+          (subscriber: Subscriber[Row]) => {
+            // For each materialization, fetch the rows from the database:
+            val rows: Publisher[Row] = databaseClient.fetchRows()
+            rows.subscribe(subscriber)
+            NotUsed
+          });
+
+    val names: Source[String, NotUsed] =
+      // rowSource can be re-used, since it will start a new
+      // query for each materialization, fully supporting backpressure
+      // for each materialized stream:
+      rowSource.map(row => row.name);
+    //#example
+}