[Doc]Add more explicit documentation for upsert and doc_as_upsert

[karenzone]

From #881:
" I think that the update a document if not already present only works with the doc_as_upsert option. The upsert option adds data to the document if the document does not exist I think.
Some more explicit documentation for both upsert and doc_as_upsert would also be helpful."

Topics:

• https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-doc_as_upsert
• https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-upsert

[karenzone]

@robin13 I added this issue for tracking your request from #881. I'd like to sync with you to hear more about the additional you'd like to see in the docs.