The `GET /_cluster/state` API returns an internal representation of the cluster
state that does change from version to version. It's useful for debugging, but
it is not intended for regular use by clients.
This change adjusts the documentation of `GET /_cluster/state` to clarify that
this API yields an internal representation that should not be expected to
remain stable between versions.
Relates #40061, #40016
8a3f87bdcd