Data lifecycle docs: Clarify ILM versus "data stream lifecycle" usage

[kilfoyle]

We should check the tutorials to make sure the the above terms are used clearly and consistently, with some explanation of the differences. We could just make sure that the main Data lifecycle page is crystal clear, and then provide links in the tutorials where those terms are used back to that main page.

Discussion:

• Data lifecycle (42:00): George: We talk about ILM and data stream lifecycle, but we
  don't really compare them. Zoya: I'd probably differentiate between Index and Data stream.
• Tutorial Update existing data stream (and others in that set) (42:30): “Data stream lifecycle policy” and “index lifecycle policy” are exactly the same thing.

(See 42:00 in recording)