Warn users about the caching implications of @MockBean and @SpyBean

[sbrannen]

The use of @MockBean and @SpyBean cause the configured test ApplicationContext to be cached under a different key in the context cache in the Spring TestContext Framework; however, this is not made apparent to users of Spring Boot's testing support, and the (potentially) undesirable side effects are not intuitive for the average user.

See related discussion on Stack Overflow.

At the very least, a warning should be included in the "Mocking and spying beans" section of the Spring Boot reference manual.

[sbrannen]

Thanks for adding the note to the reference manual!