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:

• elastic/logstash-docs#1568
• #1159.