+doc,htp #19896 directives for RequestTimeout and docs

akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala

@@ -315,8 +315,11 @@ private[http] object HttpServerBluePrint {
       requestEnd.fast.map(_ ⇒ new TimeoutSetup(Deadline.now, schedule(initialTimeout, this), initialTimeout, this))
     }
 
-    override def apply(request: HttpRequest) = HttpResponse(StatusCodes.ServiceUnavailable, entity = "The server was not able " +
+    override def apply(request: HttpRequest) =
+      //#default-request-timeout-httpresponse
+      HttpResponse(StatusCodes.ServiceUnavailable, entity = "The server was not able " +
       "to produce a timely response to your request.\r\nPlease try again in a short while!")
+      //#default-request-timeout-httpresponse
 
     def clear(): Unit = // best effort timeout cancellation
       get.fast.foreach(setup ⇒ if (setup.scheduledTask ne null) setup.scheduledTask.cancel())