Doc: Expand `silence_errors_in_log` description to show possible values

[karenzone]

TOPIC: https://www.elastic.co/guide/en/logstash/current/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-silence_errors_in_log

User request from @BBQigniter: Expand silence_errors_in_log description to show possible values. Related:

• https://github.com/elastic/logstash-docs/pull/1568
• https://github.com/logstash-plugins/logstash-output-elasticsearch/pull/1159.