Document which agents support flush intervals

https://www.elastic.co/guide/en/apm/server/master/tune-apm-server.html#reduce-payload-size

>You can reduce the payload size by decreasing the flush interval in the agents.

However, it doesn't look like all agents support flush interval. For example, Java agent no longer provides this configuration option (https://github.com/elastic/apm-agent-java/issues/571).

It will be helpful to specify the agents that are still using the older protocol in this part of the documentation (i.e., still supports flush interval configuration). Thx!