Update documentation for ES>=6.0 on sparsity for doc values & norms.

[janzankowski]

I'm filing this documentation update request as a result of this forum thread: https://discuss.elastic.co/t/in-es-6-0-is-sparsity-for-doc-values-norms-still-bad/132946

Essentially, with Lucene 7, used in ES 6.0 and up, it seems to be no longer true that if a document doesn't have a field that other documents in the index have, the field still takes disk space for doc values and norms. See http://blog.mikemccandless.com/2017/03/apache-lucene-70-is-coming-soon.html

However, this bit of documentation, for ES versions 6.0, 6.1, 6.2, and 6.3 (as of now), still says that disk space is taken: https://www.elastic.co/guide/en/elasticsearch/reference/6.0/general-recommendations.html#sparsity

If my understanding is correct, could you please update the documentation?

[elasticmachine]

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