GH-2302: Clarify in Docs the DSL log() behavior

Fixes: spring-projects/spring-integration/issues/2302

src/reference/asciidoc/dsl.adoc

@@ -1,7 +1,7 @@
 [[java-dsl]]
 == Java DSL
 
-The Spring Integration JavaConfig and DSL extension provides a set of convenient Builders and a fluent API to configure Spring Integration message flows from Spring `@Configuration` classes.
+The Spring Integration JavaConfig and DSL provides a set of convenient Builders and a fluent API to configure Spring Integration message flows from Spring `@Configuration` classes.
 
 [[java-dsl-example]]
 === Example Configurations
@@ -491,10 +491,43 @@ public IntegrationFlow loggingFlow() {
 }
 ----
 
+[IMPORTANT]
+====
+The `log()` or `wireTap()` opearators are applied to the current `MessageChannel` (if it is an instance of `ChannelInterceptorAware`) or an intermediate `DirectChannel` is injected into the flow for the currently configured endpoint.
+In the example above the `WireTap` interceptor is added to the `myChannel` directly, because `QueueChannel` implements `ChannelInterceptorAware`:
+====
+
+[source,java]
+----
+@Bean
+MessageChannel myChannel() {
+    return new DirectChannel();
+}
+
+...
+    .channel(myChannel())
+    .log()
+}
+----
+
+When current `MessageChannel` doesn't implement `ChannelInterceptorAware`, an implicit `DirectChannel` and `BridgeHandler` are injected into the `IntegrationFlow` and the `WireTap` is added to this new `DirectChannel`.
+And when there is not any channel declaration like in this sample:
+
+[source,java]
+----
+.handle(...)
+.log()
+}
+----
+
+an implicit `DirectChannel` is injected in the current position of the `IntegrationFlow` and it is used as an output channel for the currently configured `ServiceActivatingHandler` (the `.handle()` above).
+
 [IMPORTANT]
 =====
-If `log()` or `wireTap()` are used in the end of flow they are considered as one-way `MessageHandler` s.
-If the integration flow is expected to return reply, the `bridge()` should be used in the end, after `log()` or `wireTap()`:
+If `log()` or `wireTap()` are used in the end of flow they are considered one-way `MessageHandler` s.
+If the integration flow is expected to return a reply, a `bridge()` should be added to the end, after `log()` or `wireTap()`:
+=====
+
 [source,java]
 ----
 @Bean
@@ -509,7 +542,6 @@ public IntegrationFlow sseFlow() {
         .get();
 }
 ----
-=====
 
 [[java-dsl-flows]]
 === Working With Message Flows