INT-4257-2: Document ErrorMessagePublisher

JIRA: https://jira.spring.io/browse/INT-4257

* Remove deprecated `EnhancedErrorMessage`
* Document `ErrorMessagePublisher` and `ErrorMessageStrategy`
* Fix Reactor version to `3.1` in Doc
* Fix sample about Reactive Streams in the `router.adoc`

src/reference/asciidoc/configuration.adoc

@@ -195,6 +195,15 @@ To enable global error handling, simply register a handler on that channel.
 For example, you can configure Spring Integration's `ErrorMessageExceptionTypeRouter` as the handler of an endpoint that is subscribed to the 'errorChannel'.
 That router can then spread the error messages across multiple channels based on `Exception` type.
 
+Starting with _version 4.3.10_, the `ErrorMessagePublisher` and the `ErrorMessageStrategy` are provided.
+They can be used as general mechanism for publishing `ErrorMessage` s and can be called or extended in any error handling scenarios.
+The `ErrorMessageSendingRecoverer` extends this class as a `RecoveryCallback` implementation that can be used with retry, such as the
+<<retry-advice, RequestHandlerRetryAdvice>>.
+The `ErrorMessageStrategy` is used to build an `ErrorMessage` based on the provided exception and an `AttributeAccessor` context.
+It can be injected to any `MessageProducerSupport` and `MessagingGatewaySupport` - and the `requestMessage` is stored under `ErrorMessageUtils.INPUT_MESSAGE_CONTEXT_KEY` in the `AttributeAccessor` context.
+The `ErrorMessageStrategy` can use that `requestMessage` as the `originalMessage` property of the `ErrorMessage` it creates.
+The `DefaultErrorMessageStrategy` does exactly that.
+
 [[global-properties]]
 === Global Properties
 

src/reference/asciidoc/router.adoc

@@ -1380,18 +1380,12 @@ public PollableChannel resultsChannel() {
 public RoutingSlipRouteStrategy routeStrategy() {
     return (requestMessage, reply) -> requestMessage.getPayload() instanceof String
             ? new FixedSubscriberChannel(m ->
-            Streams.defer((String) m.getPayload())
-                    .env(this.reactorEnv)
-                    .get()
+            Mono.just((String) m.getPayload())
                     .map(String::toUpperCase)
-                    .consume(v -> messagingTemplate().convertAndSend(resultsChannel(), v))
-                    .flush())
+                    .subscribe(v -> messagingTemplate().convertAndSend(resultsChannel(), v)))
             : new FixedSubscriberChannel(m ->
-            Streams.defer((Integer) m.getPayload())
-                    .env(this.reactorEnv)
-                    .get()
+            Mono.just((Integer) m.getPayload())
                     .map(v -> v * 2)
-                    .consume(v -> messagingTemplate().convertAndSend(resultsChannel(), v))
-                    .flush());
+                    .subscribe(v -> messagingTemplate().convertAndSend(resultsChannel(), v)));
 }
 ----

src/reference/asciidoc/whats-new.adoc

@@ -37,11 +37,15 @@ See <<http-outbound>> for more information.
 Now that we use the new `InvocableHandlerMethod` -based infrastructure for service method invocations, we can perform `contentType` conversion from payload to target method argument.
 See <<content-type-conversion>> for more information.
 
+==== ErrorMessagePublisher and ErrorMessageStrategy
+
+The `ErrorMessagePublisher` abd the `ErrorMessageStrategy` are provided for creating `ErrorMessage` instances.
+See <<namespace-errorhandler>> for more information.
 
 [[x5.0-general]]
 === General Changes
 
-Spring Integration is now fully based on Spring Framework `5.0` and Project Reactor `3.0`.
+Spring Integration is now fully based on Spring Framework `5.0` and Project Reactor `3.1`.
 Previous Project Reactor versions are no longer supported.
 
 ==== Core Changes