Add tip about using AopTestUtils.getTargetProxy with proxied spy beans

Closes gh-22281

Author: wilkinsona

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

@@ -6286,6 +6286,9 @@ NOTE: While Spring's test framework caches application contexts between tests an
 TIP: If you are using `@SpyBean` to spy on a bean with `@Cacheable` methods that refer to parameters by name, your application must be compiled with `-parameters`.
 This ensures that the parameter names are available to the caching infrastructure once the bean has been spied upon.
 
+TIP: When you are using `@SpyBean` to spy on a bean that is proxied by Spring, you may need to remove Spring's proxy in some situations, for example when setting expectations using `given` or `when`.
+Use `AopTestUtils.getTargetProxy(yourProxiedSpy)` to do so.
+
 
 
 [[boot-features-testing-spring-boot-applications-testing-autoconfigured-tests]]