Improve documentation of when TestRestTemplate is auto-configured

Closes gh-6729

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -5488,9 +5488,10 @@ public class MyTest {
 }
 ----
 
-If you are using the `@SpringBootTest` annotation, you can just inject a fully configured
-`TestRestTemplate` and start using it. If necessary, additional customizations can be
-applied via the `RestTemplateBuilder` bean:
+If you are using the `@SpringBootTest` annotation with `WebEnvironment.RANDOM_PORT` or
+`WebEnvironment.DEFINED_PORT`, you can just inject a fully configured `TestRestTemplate`
+and start using it. If necessary, additional customizations can be applied via the
+`RestTemplateBuilder` bean:
 
 [source,java,indent=0]
 ----

spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTest.java

@@ -55,7 +55,8 @@ import org.springframework.web.context.WebApplicationContext;
  * {@link WebEnvironment#DEFINED_PORT defined} or {@link WebEnvironment#RANDOM_PORT
  * random} port.</li>
  * <li>Registers a {@link org.springframework.boot.test.web.client.TestRestTemplate
- * TestRestTemplate} bean for use in web tests.</li>
+ * TestRestTemplate} bean for use in web tests that are using a fully running container
+ * </li>
  * </ul>
  *
  * @author Phillip Webb