Whether the cold tier can handle years depends a lot on the use case and
for instance our BWC guarantees. This would need to be part of a
specific sizing exercise, so in the spirit of not over-promising, the
description of the cold tier has been changed to not mention years.
This adds general overview documentation for data tiers,
the data tiers specific node roles, and their application in
ILM.
Co-authored-by: Lee Hinman <dakrone@users.noreply.github.com>
Co-authored-by: debadair <debadair@elastic.co>