Doc use of reactive types in Spring MVC controllers

Issue: SPR-15365

spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/AsyncSupportConfigurer.java

@@ -56,6 +56,11 @@ public class AsyncSupportConfigurer {
 	 * highly recommended to change that default in production since the simple
 	 * executor does not re-use threads.
 	 *
+	 * <p>As of 5.0 this executor is also used when a controller returns a reactive
+	 * type that does streaming (e.g. "text/event-stream" or
+	 * "application/stream+json") for the blocking writes to the
+	 * {@link javax.servlet.ServletOutputStream}.
+	 *
 	 * @param taskExecutor the task executor instance to use by default
 	 */
 	public AsyncSupportConfigurer setTaskExecutor(AsyncTaskExecutor taskExecutor) {

spring-webmvc/src/main/resources/org/springframework/web/servlet/config/spring-mvc.xsd

@@ -231,9 +231,14 @@
 						<xsd:attribute name="task-executor" type="xsd:string">
 							<xsd:annotation>
 								<xsd:documentation source="java:org.springframework.core.task.AsyncTaskExecutor"><![CDATA[
-	The bean name of a default AsyncTaskExecutor to use when a controller method returns a {@link Callable}.
+	The bean name of a default AsyncTaskExecutor to use when a controller method returns a "Callable".
 	Controller methods can override this default on a per-request basis by returning an AsyncTask.
 	By default, a SimpleAsyncTaskExecutor is used which does not re-use threads and is not recommended for production.
+
+	As of 5.0 this executor is also used when a controller returns a reactive type that does streaming
+	(e.g. "text/event-stream" or "application/stream+json") for the blocking writes to the
+	"javax.servlet.ServletOutputStream".
+
 								]]></xsd:documentation>
 								<xsd:appinfo>
 									<tool:annotation kind="ref">