Fixing documentation inconsistencies

This pull request fix some inconsistencies found in the documentation, like the ones listed below:

* Confusing message in the getting started tutorial vs the logstash-output-elasticsearch pages. Might be a good idea to drop a link here to this page, but sure how to do that. Issue #2098 
* References to logstash.net what should be move to elasticsearch site. [WiP]

I meant this file https://github.com/elasticsearch/logstash-docs/blob/master/docs/static/configuration.asciidoc

Basically link 'plugin configuration" to point to this page