Improve documentation for `TestInstantiationAwareExtension` and raise awareness of semantic differences

[sbrannen]

Overview

While working on spring-projects/spring-framework#35697, I noticed that the documentation for TestInstantiationAwareExtension could be improved by including additional affected use cases, such as how to access the "current test class" in a TestInstancePostProcessor or when resolving a parameter for a constructor in a ParameterResolver.

Related Issues

• Use test-method scoped ExtensionContext in the SpringExtension spring-projects/spring-framework#35697