Docs: API docs for asSourceWithContext (#28950)

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

@@ -4312,6 +4312,9 @@ final class Source[Out, Mat](delegate: scaladsl.Source[Out, Mat]) extends Graph[
   def logWithMarker(name: String, marker: function.Function[Out, LogMarker]): javadsl.Source[Out, Mat] =
     this.logWithMarker(name, marker, ConstantFun.javaIdentityFunction[Out], null)
 
+  /**
+   * Transform this source whose element is ``e`` into a source producing tuple ``(e, f(e))``
+   **/
   def asSourceWithContext[Ctx](extractContext: function.Function[Out, Ctx]): SourceWithContext[Out, Ctx, Mat] =
     new scaladsl.SourceWithContext(this.asScala.map(x => (x, extractContext.apply(x)))).asJava
 }