Improve RestClientException Javadoc

Closes gh-24288

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,10 +17,13 @@
 package org.springframework.web.client;
 
 import org.springframework.core.NestedRuntimeException;
+import org.springframework.http.client.ClientHttpResponse;
 
 /**
- * Base class for exceptions thrown by {@link RestTemplate} whenever it encounters
- * client-side HTTP errors.
+ * Base class for exceptions thrown by {@link RestTemplate} in case a request
+ * fails because of a server error response, as determined via
+ * {@link ResponseErrorHandler#hasError(ClientHttpResponse)}, failure to decode
+ * the response, or a low level I/O error.
  *
  * @author Arjen Poutsma
  * @since 3.0