Logstash: Review plugin doc PRs

[dedemorton]

These PRs need to be reviewed:

• [x] https://github.com/logstash-plugins/logstash-input-http_poller/pull/85
• [x] https://github.com/logstash-plugins/logstash-output-rabbitmq/pull/70
• [x] https://github.com/logstash-plugins/logstash-input-dead_letter_queue/pull/13
• [x] https://github.com/logstash-plugins/logstash-input-elasticsearch/pull/77
• [x] https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/671
• [x] https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/670
• [x] https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/677
• [x] https://github.com/logstash-plugins/logstash-input-http_poller/pull/86

(added backports to make it easier for you to review)

[ph]

logstash-input-http_poller need backport to 3.x
logstash-output-rabbitmq need backport to 4.x
logstash-output-elasticsearch 7.x

[dedemorton]

Backported changes in logstash-input-http_poller to 3.x and logstash-output-elasticsearch to 7x .

The problem in logstash-output-rabbitmq is already fixed in 4.x, so I didn't have to backport.

[ph]

@dedemorton all of theses changes are reviewed, merged and published.