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:

• Update silence_errors_in_log example elastic/logstash-docs#1568
• Replace document_already_exist_exception with version_conflict_engine_exception in the silence_errors_in_log setting example #1159.