Update documentation to reflect @AutoConfigureRestDocs support for @WebTestClient

[The reference documentation section on restdocs](https://docs.spring.io/spring-boot/docs/2.1.0.RELEASE/reference/htmlsingle/#boot-features-testing-spring-boot-applications-testing-autoconfigured-rest-docs) states that `@AutoConfigureRestDocs` only supports REST Assured and Mock MVC. Thanks to  https://github.com/spring-projects/spring-boot/pull/10969/, `WebTestClient` is also supported.