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]

can we link to the "plugin configuration" section using `<<plugin_configuration>>`.

For this we need to first add a reference in static/plugin_configuration.asciidoc .

See for example: https://raw.githubusercontent.com/elasticsearch/logstash-docs/master/docs/plugins/inputs/elasticsearch.asciidoc