Document that test slices should not be combined

See gh-15310

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

@@ -6837,6 +6837,10 @@ restricted set of auto-configuration classes. If you need to exclude one of them
 most `@...Test` annotations provide an `excludeAutoConfiguration` attribute.
 Alternatively, you can use `@ImportAutoConfiguration#exclude`.
 
+NOTE: Including multiple "`slices`" via the several `@...Test` annotations in one test is
+not supported. If you need multiple "`slices`", pick one of the `@...Test` annotations
+and include the `@AutoConfigure...` annotations of the other "`slices`" by hand.
+
 TIP: It is also possible to use the `@AutoConfigure...` annotations with the standard
 `@SpringBootTest` annotation. You can use this combination if you are not interested in
 "`slicing`" your application but you want some of the auto-configured test beans.