Improve EnableCaching & CachingConfigurer Javadoc

[snicoll]

Feedback on #33282 revealed that the Javadoc of the caching infra need some polish.

For instance, it is still claiming that keyGenerator is required (because that piece of the doc was written before we could use default methods in interfaces). And the bean requirement is not really obvious either for keygenerator and errorHandler. We should rather explain why it's important for lifecycle purposes.