Add reference to ConcurrentWebSocketSessionDecorator in the docs.

Issue: SPR-12886

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/WebSocketSession.java

@@ -120,6 +120,13 @@ public interface WebSocketSession extends Closeable {
 
 	/**
 	 * Send a WebSocket message: either {@link TextMessage} or {@link BinaryMessage}.
+	 *
+	 * <p><strong>Note:</strong> The underlying standard WebSocket session (JSR-356) does
+	 * not allow concurrent sending. Therefore sending must be synchronized. To ensure
+	 * that, one option is to wrap the {@code WebSocketSession} with the
+	 * {@link org.springframework.web.socket.handler.ConcurrentWebSocketSessionDecorator
+	 * ConcurrentWebSocketSessionDecorator}.
+	 * @see org.springframework.web.socket.handler.ConcurrentWebSocketSessionDecorator
 	 */
 	void sendMessage(WebSocketMessage<?> message) throws IOException;
 

src/docs/asciidoc/web/websocket.adoc

@@ -102,12 +102,18 @@ The following example shows the XML configuration equivalent of the preceding ex
 ----
 ====
 
-The pereceding example is for use in Spring MVC applications and should be included in the
+The preceding example is for use in Spring MVC applications and should be included in the
 configuration of a <<mvc-servlet,`DispatcherServlet`>>. However, Spring's WebSocket
 support does not depend on Spring MVC. It is relatively simple to integrate a `WebSocketHandler`
 into other HTTP-serving environments with the help of
 {api-spring-framework}/web/socket/server/support/WebSocketHttpRequestHandler.html[`WebSocketHttpRequestHandler`].
 
+When using the `WebSocketHandler` API directly vs indirectly, e.g. through the
+<<websocket-stomp>> messaging, the application must synchronize the sending of messages
+since the underlying standard WebSocket session (JSR-356) does not allow concurrent
+sending. One option is to wrap the `WebSocketSession` with
+{api-spring-framework}/web/socket/handler/ConcurrentWebSocketSessionDecorator.html[`ConcurrentWebSocketSessionDecorator`].
+
 
 
 [[websocket-server-handshake]]