Document load balance option for Elasticsearch output

[ppf2]

https://www.elastic.co/guide/en/apm/server/current/elasticsearch-output.html#_worker

This is best used with load balancing mode enabled.

But I don't see the load balancer option documented anywhere within the APM guide.

I suspect that this is because we pulled the ES output option descriptions from the Beats documentation, except that Beats has its own documentation page that talks about the load balancer option under their "Configure" section (https://www.elastic.co/guide/en/beats/filebeat/current/load-balancing.html). In our APM guide, we do not have this load balancing subsection under "Configure" so this information is missing for APM.