SpringBoot Document that ReactiveElasticsearchTemplate is configured to immediate refresh

We should document that the ReactiveElasticsearchTemplate is configured in our auto-configuration to set refresh to IMMEDIATE, which can cause performance problems.

See https://github.com/spring-projects/spring-boot/issues/30096

Comment From: mhalbritter

After a discussion we decided to not document the current behaviour. #30096 will set the elastic template to spring data's defaults, and they should be documented there.