Spring Correct inaccuracy in spring docs

Spring uses not only BeanPostProcessor-s to process callback interfaces. For example AbstractAutowireCapableBeanFactory#initializeBean is calling afterPropertiesSet on the InitializingBean-s.

Verified for at least spring-beans v6.0.10

Comment From: snicoll

@luchob the intention of the paragraph is to explain that the framework uses BeanPostProcessor internally to implement the features exposed there, and that users can implement custom callback with a similar mechanism. While you are correct, I don't see a value of providing additional details that serve no purpose in the context of that section.

Thanks for the PR, in any case!