[DOCS] initial_position_in_stream is not documented in official site.

[dharada]

We can't see the description about initial_position_in_stream option.

• https://www.elastic.co/guide/en/logstash/current/plugins-inputs-kinesis.html

but We can find the description here.

• https://github.com/logstash-plugins/logstash-input-kinesis

（Request) I think it should be documented as official document.