* Deprecate single-tier allocation filtering settings
`(index|cluster).routing.allocation.(include|exclude|require)._tier` settings are now deprecated in
favor of using `index.routing.allocation.include._tier_preference`.
* Update deprecation message
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>