Update documentation regarding nested test class support

[sbrannen]

Overview

With the introduction of @NestedTestConfiguration in 5.3, we introduced first-class support for inheriting test configuration in @Nested test classes, but the documentation was not consistently updated to reflect this.

Related Issues

• Discover test configuration on enclosing class for nested test class [SPR-15366] #19930
• Provide guidance on using @Nested within test classes with complex inheritance and annotation arrangements #28466