The `GET _cluster/state` API is really only suitable for debugging or
diagnostics. Its response format is not documented since it changes
fairly freely between versions.
Today we mention in its docs that this API is unstable, and deliberately
omit a description of its response format, but we don't explicitly say
that it's only for diagnostics and is unsuitable for consumption by
external tools that might try and use it for monitoring.
This commit adjusts the docs to give some more explicit guidance about
how it should and shouldn't be used.
dc26886ee1