Document that `TestContextAnnotationUtils` is required for proper `@Nested` test class support

[sbrannen]

Overview

As @artembilan mentioned in spring-projects/spring-integration#9506 (review), it's not readily apparent from the documentation for @NestedTestConfiguration that one must use TestContextAnnotationUtils in order to properly support annotation lookups for @Nested test classes, and we should document that.

Related Issues

• Better inheritance support for @Nested when using @SpringIntegration spring-integration#9492
• Improve support for @Nested and @NestedTestConfiguration in various test framework contracts #30310
• Configuration from enclosing class not discovered for @Nested test class when enclosing class is not annotated with @ContextConfiguration #31456