Document the need to explicitly reset mock servers

When MockServerRestTemplateCustomizer and MockServerRestClientCustomizer
are used directly instead of via auto-configuration, it is necessary to
manually reset the MockRestServiceServer expectations between tests.

Closes gh-41848

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/MockServerRestClientCustomizer.java

@@ -50,6 +50,10 @@ import org.springframework.web.client.RestClient.Builder;
  * obtain the mock server. If the customizer has been used more than once the
  * {@link #getServer(RestClient.Builder)} or {@link #getServers()} method must be used to
  * access the related server.
+ * <p>
+ * If a mock server is used in more than one test case in a test class, it might be
+ * necessary to reset the expectations on the server between tests using
+ * {@code getServer().reset()} or {@code getServer(restClientBuilder).reset()}.
  *
  * @author Scott Frederick
  * @since 3.2.0

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/web/client/MockServerRestTemplateCustomizer.java

@@ -48,6 +48,10 @@ import org.springframework.web.client.RestTemplate;
  * obtain the mock server. If the customizer has been used more than once the
  * {@link #getServer(RestTemplate)} or {@link #getServers()} method must be used to access
  * the related server.
+ * <p>
+ * If a mock server is used in more than one test case in a test class, it might be
+ * necessary to reset the expectations on the server between tests using
+ * {@code getServer().reset()} or {@code getServer(restTemplate).reset()}.
  *
  * @author Phillip Webb
  * @author Moritz Halbritter