[Doc][Meta] Improvements to output-elasticsearch docs

[karenzone]

Improvements to documentation

• Do a better job connecting the dots wrt secure communication with ES. The details are accurate, but we're missing out by not elevating key concepts. Also, explain how ESS simplifies security to ES.
• Request for "more explicit documentation for both upsert and doc_as_upsert" in [Doc]Add more explicit documentation for upsert and doc_as_upsert #928
• [Doc] Update info for validate_after_inactivity option #1017
• timeout - Line 639. Missing word. "requests sent Elasticsearch" should to "requests sent to Elasticsearch."
• Some config options are out of order. Fix the order.
• Add examples:
  • cacert option (and others)

Questions/issues to resolve

• http compression - Line 391.
  • Text: "Note that response compression is on by default for Elasticsearch v5.0 and beyond," but default value shows false.
  • Proposed resolution: Change documented default to true and delete note.
• sniffing - Line 587
  • Link: https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-sniffing
  • Does the behavior we list for Elasticsearch 5.x and 6.x extend to 7.x and beyond?
  • Proposed resolution:
    • Delete references to 1.x and 2.x behaviors.
    • If 5.x and 6.x behavior (excluding master-only nodes) extends to 7.x and beyond, delete references to specific versions and document that as the default behavior.

This issue is a continuation of #885