GH-3800: Fix Kotlin docs for route() DSL

Fixes #3800

* Mention `Message` as a type for reified generic argument in Kotlin DSL docs

**Cherry-pick to `5.5.x`**

spring-integration-core/src/main/kotlin/org/springframework/integration/dsl/KotlinIntegrationFlowDefinition.kt

@@ -145,15 +145,15 @@ class KotlinIntegrationFlowDefinition(@PublishedApi internal val delegate: Integ
 
 
 	/**
-	 * Inline function for [IntegrationFlowDefinition.filter] providing a `filter<MyTypeIn>()` variant
+	 * Inline function for [IntegrationFlowDefinition.route] providing a `route<MyTypeIn>()` variant
 	 * with reified generic type.
 	 */
 	inline fun <reified P> route(crossinline function: (P) -> Any?) {
 		route(function) { }
 	}
 
 	/**
-	 * Inline function for [IntegrationFlowDefinition.filter] providing a `filter<MyTypeIn>()` variant
+	 * Inline function for [IntegrationFlowDefinition.route] providing a `route<MyTypeIn>()` variant
 	 * with reified generic type.
 	 */
 	inline fun <reified P, T> route(

src/reference/asciidoc/kotlin-dsl.adoc

@@ -93,4 +93,5 @@ fun convertFlow() =
 ----
 ====
 
+NOTE: The reified type can be a whole `Message<*>` if there need access to headers as well in the lambda of the operator.