GH-2707 Add PostProcessing documentation

Resolves #2707

Author: olegz

docs/src/main/asciidoc/spring-cloud-stream.adoc

@@ -1485,6 +1485,80 @@ NOTE: If you need to support dynamic destinations with multiple binder types, us
 
 Also, please see <<Using StreamBridge>> section to see how yet another option (StreamBridge) can be utilized for similar cases.
 
+=== Post processing (after sending message)
+
+Once function is invoked, its result is sent by the framework to a target destination which effectively completes function invocation cycle. 
+
+However such cycle may not be fully complete from the business standpoint until some additional tasks are performed *after* completion of this cycle. 
+While this could be accomplished with a simple combination of `Consumer` and `StreamBridge` as described in this 
+https://stackoverflow.com/questions/75917883/post-processing-after-spring-cloud-stream-function[Stack Overflow post], since version 4.0.3 the framework 
+provides a more idiomatic approach to solve this issue via `PostProcessingFunction` provided by Spring Cloud Function project.
+The `PostProcessingFunction` is a special semi-marker function which contains one additional method `postProcess(Message>)` designed 
+to provide a place for implementing such post processing task. 
+
+----
+package org.springframework.cloud.function.context
+. . .
+public interface PostProcessingFunction<I, O> extends Function<I, O> {
+	default void postProcess(Message<O> result) {
+	}
+}
+----
+
+So, now you have two options.
+
+Option 1: You can implement your function as `PostProcessingFunction` and also include the additional post processing behavior by implementing its `postProcess(Message>)` method.
+
+----
+private static class Uppercase implements PostProcessingFunction<String, String> {
+
+	@Override
+	public String apply(String input) {
+		return input.toUpperCase();
+	}
+
+	@Override
+	public void postProcess(Message<String> result) {
+		System.out.println("Function Uppercase has been successfully invoked and its result successfully sent to target destination");
+	}
+}
+. . .
+@Bean
+public Function<String, String> uppercase() {
+	return new Uppercase();
+}
+----
+
+Option 2: If you already have an existing function and don't want to change its implementation or want to keep your function as POJO, you can simply implement only `postProcess(Message>)` method and compose this new post processing function with your other function.
+----
+private static class Logger implements PostProcessingFunction<?, String> {
+
+	@Override
+	public void postProcess(Message<String> result) {
+		System.out.println("Function has been successfully invoked and its result successfully sent to target destination");
+	}
+}
+. . .
+@Bean
+public Function<String, String> uppercase() {
+	return v -> v.toUpperCase();
+}
+@Bean
+public Function<String, String> logger() {
+	return new Logger();
+}
+. . .
+//  and then have your function definition as such `uppercase|logger`
+----
+
+NOTE: 
+In case of function composition only the last instance of `PostProcessingFunction` (if present) will take effect. For example, let's say you have the 
+following function definition - `foo|bar|baz` and both `foo` and `baz` are instances of `PostProcessingFunction`. Only `baz.postProcess(Message>)` will be invoked.
+If `baz` is not an instance of `PostProcessingFunction`, then no post processing functionality will be performed. 
+
+One may argue that you can easily do that via function composition by simply composing a post-processor as just another `Function`. That is indeed a possibility however 
+the post processing functionality in this case will be invoked right after invocation of the previous function and before the message is sent to a target destination
+which is before the function invocation cycle is complete.
 
 [[spring-cloud-stream-overview-error-handling]]
 === Error Handling