Improve documentation on Event API

The docs on the [Event API](https://www.elastic.co/guide/en/logstash/current/event-api.html) are pretty incomplete and users have to [read through the code](https://github.com/elastic/logstash/blob/master/logstash-core/src/main/java/org/logstash/Event.java) to understand what's going on.

We should provide better documentation for this to make is easier for users to contribute.