Document `@ImportAutoConfiguration#exclude`

Closes gh-7862

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/ImportAutoConfiguration.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,9 @@ import org.springframework.core.annotation.AliasFor;
  * rules as {@code @EnableAutoConfiguration} but restricts the auto-configuration classes
  * to the specified set, rather than consulting {@code spring.factories}.
  * <p>
+ * Can also be used to {@link #exclude()} specific auto-configuration classes such that
+ * they will never be applied.
+ * <p>
  * Generally, {@code @EnableAutoConfiguration} should be used in preference to this
  * annotation, however, {@code @ImportAutoConfiguration} can be useful in some situations
  * and especially when writing tests.

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

@@ -5383,6 +5383,10 @@ providing a `@...Test` annotation that loads the `ApplicationContext` and one or
 more `@AutoConfigure...` annotations that can be used to customize auto-configuration
 settings.
 
+NOTE: Each slice loads a very restricted set of auto-configuration classes. If you need to
+exclude one of them, most `@...Test` annotation provide an `excludeAutoConfiguration`
+attribute. Alternatively, you can use `@ImportAutoConfiguration#exclude`.
+
 TIP: It's also possible to use the `@AutoConfigure...` annotations with the standard
 `@SpringBootTest` annotation. You can use this combination if you're not interested
  in '`slicing`' your application but you want some of the auto-configured test beans.