Document using @TestConfiguration to customize app's primary config

Closes gh-5566

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

@@ -4442,14 +4442,19 @@ to load. Alternatively, you might have often used nested `@Configuration` classe
 your test.
 
 When testing Spring Boot applications this is often not required.
-Spring Boot's `@*Test` annotations will search for your primary configuration automatically
-whenever you don't explicitly defined one.
+Spring Boot's `@*Test` annotations will search for your primary configuration
+automatically whenever you don't explicitly define one.
 
 The search algorithm works up from the package that contains the test until it finds a
 `@SpringBootApplication` or `@SpringBootConfiguration` annotated class. As long as you've
 <<using-boot-structuring-your-code, structure your code>> in a sensible way your main
 configuration is usually found.
 
+If you want to customize the primary configuration, you can use a nested
+`@TestConfiguration` class. Unlike a nested `@Configuration` class which would be used
+instead of a your application's primary configuration, a nested `@TestConfiguration` class
+will be used in addition to your application's primary configuration.
+
 NOTE: Spring's test framework will cache application contexts between tests. Therefore, as
 long as your tests share the same configuration (no matter how it's discovered), the
 potentially time consuming process of loading the context will only happen once.