=str #16608 adds scaladoc about null in col returned from mapConcat

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

@@ -292,6 +292,9 @@ class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[Sour
   /**
    * Transform each input element into a sequence of output elements that is
    * then flattened into the output stream.
+   *
+   * The returned list MUST NOT contain `null` values,
+   * as they are illegal as stream elements - according to the Reactive Streams specification.
    */
   def mapConcat[T](f: japi.Function[Out, java.util.List[T]]): javadsl.Source[T, Mat] =
     new Source(delegate.mapConcat(elem ⇒ Util.immutableSeq(f.apply(elem))))