Update javadoc for (Async)RestTemplate

Issue: SPR-13313

spring-web/src/main/java/org/springframework/http/client/support/HttpAccessor.java

@@ -50,7 +50,8 @@ public abstract class HttpAccessor {
 
 
 	/**
-	 * Set the request factory that this accessor uses for obtaining {@link ClientHttpRequest HttpRequests}.
+	 * Set the request factory that this accessor uses for obtaining
+	 * {@link ClientHttpRequest HttpRequests}.
 	 */
 	public void setRequestFactory(ClientHttpRequestFactory requestFactory) {
 		Assert.notNull(requestFactory, "'requestFactory' must not be null");

spring-web/src/main/java/org/springframework/web/client/AsyncRestTemplate.java

@@ -63,6 +63,11 @@ import org.springframework.web.util.UriTemplateHandler;
  * {@linkplain #setMessageConverters(List) message converters} with this
  * {@code RestTemplate}.
  *
+ * <p><strong>Note:</strong> by default {@code AsyncRestTemplate} relies on
+ * standard JDK facilities to establish HTTP connections. You can switch to use
+ * a different HTTP library such as Apache HttpComponents, Netty, and OkHttp by
+ * using a constructor accepting an {@link AsyncClientHttpRequestFactory}.
+ *
  * <p>For more information, please refer to the {@link RestTemplate} API documentation.
  *
  * @author Arjen Poutsma

spring-web/src/main/java/org/springframework/web/client/RestTemplate.java

@@ -60,6 +60,11 @@ import org.springframework.web.util.UriTemplateHandler;
  * It handles HTTP connections, leaving application code to provide URLs
  * (with possible template variables) and extract results.
  *
+ * <p><strong>Note:</strong> by default the RestTemplate relies on standard JDK
+ * facilities to establish HTTP connections. You can switch to use a different
+ * HTTP library such as Apache HttpComponents, Netty, and OkHttp through the
+ * {@link #setRequestFactory} property.
+ *
  * <p>The main entry points of this template are the methods named after the six main HTTP methods:
  * <table>
  * <tr><th>HTTP method</th><th>RestTemplate methods</th></tr>