Add resource usage documentation for ElasticSearchClientServiceImpl

We observed a significant resource usage when using many ES controller service instances. After investigation, it appears that the ES client relies on a RestClient where the Apache Http Async Client starts by default one dispatcher thread, and a number of worker threads used by the connection manager, as many as the number of locally detected processors (depending on what Runtime.getRuntime().availableProcessors() returns).

I think it'd be worth adding this in the documentation and the proper annotation where relevant.