INT-3538: Docs for Routing Slip

JIRA: https://jira.spring.io/browse/INT-3538

INT-3538-2: `RoutingSlipHeaderValueMessageProcessor` improvement

Since `HeaderEnricher` populates `beanFactory` to its `HeaderValueMessageProcessor`s,
there is no reason to specify `RoutingSlipHeaderValueMessageProcessor` as a separate bean.
Therefore the `IntegrationEvaluationContextAware` may not be reached.
Remove its support from `RoutingSlipHeaderValueMessageProcessor` and just rely on the `setBeanFactory`

RS Doc Polishing