Simple Documentation Improvement - Bridging the gap between FluentD and ElasticSearch

### Describe the bug

The documentation for the Kubernetes DaemonSet [here](https://docs.fluentd.org/v/0.12/articles/kubernetes-fluentd) is _very well written_, but I would like to suggest that it includes a couple of sentences bridging the gap between the FluentD ElasticSearch integration and ElasticSearch itself. There is no mention of how this data is passed to ElasticSearch, which makes it difficult to know where to pick up in the ElasticSearch documentation.

### Link to the problematic documentation

https://docs.fluentd.org/v/0.12/articles/kubernetes-fluentd

### Expected explanation

A couple of sentences mentioning that the ElasticSearch plugin ships logs to an ElasticSearch cluster using the [Elastic Index API](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-index_.html). This makes it clear how the plugin functions, and where a reader might go to learn more.

### Additional context

The ElasticSearch Index API is a guess based on the full daemonset manifest, but I think that just proves my point. It's an easy addition that noticeably improves the quality of the documentation.