Clarify cluster state API docs (#82930)

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.

docs/reference/cluster/state.asciidoc

@@ -4,7 +4,8 @@
 <titleabbrev>Cluster state</titleabbrev>
 ++++
 
-Returns metadata about the state of the cluster.
+Returns an internal representation of the cluster state for debugging or
+diagnostic purposes.
 
 [[cluster-state-api-request]]
 ==== {api-request-title}
@@ -20,44 +21,46 @@ Returns metadata about the state of the cluster.
 [[cluster-state-api-desc]]
 ==== {api-description-title}
 
-The cluster state API allows access to metadata representing the state of the
-whole cluster. This includes information such as
+The _cluster state_ is an internal data structure which keeps track of a
+variety of information needed by every node, including:
 
-* the set of nodes in the cluster
+* The identity and attributes of the other nodes in the cluster
 
-* all cluster-level settings
+* Cluster-wide settings
 
-* information about the indices in the cluster, including their mappings and
-  settings
+* Index metadata, including the mapping and settings for each index
 
-* the locations of all the shards in the cluster.
+* The location and status of every shard copy in the cluster
 
-NOTE: The response is an internal representation of the cluster state and its
-format may change from version to version. If possible, you should obtain any
-information from the cluster state using the other, more stable,
-<<cluster,cluster APIs>>.
+The elected master node ensures that every node in the cluster has a copy of
+the same cluster state. The cluster state API lets you retrieve a
+representation of this internal state for debugging or diagnostic purposes. You
+may need to consult the {es} source code to determine the precise meaning of
+the response.
 
-The response provides the cluster state itself, which can be filtered to only
-retrieve the parts of interest as described below.
+By default the cluster state API will route requests to the elected master node
+since this node is the authoritative source of cluster states. You can also
+retrieve the cluster state held on the node handling the API request by adding
+the query parameter `?local=true`.
 
-The cluster's `cluster_uuid` is also returned as part of the top-level response,
-in addition to the `metadata` section.
+{es} may need to expend significant effort to compute a response to this API in
+larger clusters, and the response may comprise a very large quantity of data.
+If you use this API repeatedly, your cluster may become unstable.
 
-NOTE: While the cluster is still forming, it is possible for the `cluster_uuid`
-      to be `_na_` as well as the cluster state's version to be `-1`.
-
-By default, the cluster state request is routed to the master node, to ensure
-that the latest cluster state is returned. For debugging purposes, you can
-retrieve the cluster state local to a particular node by adding `local=true` to
-the query string.
+[[cluster-state-api-unstable-warning]]
+WARNING: The response is a representation of an internal data structure. Its
+format is not subject to the same compatibility guarantees as other more stable
+APIs and may change from version to version. **Do not query this API using
+external monitoring tools.** Instead, obtain the information you require using
+other more stable <<cluster,cluster APIs>>.
 
 [[cluster-state-api-path-params]]
 ==== {api-path-parms-title}
 
-The cluster state contains information about all the indices in the cluster,
-including their mappings, as well as templates and other metadata. This means it
-can sometimes be quite large. To avoid the need to process all this information
-you can request only the part of the cluster state that you need:
+The cluster state can sometimes be very large, and {es} may consume significant
+resources while computing a response to this API. To reduce the size of the
+response, you can request only the part of the cluster state in which you are
+interested:
 
 `<metrics>`::
     (Optional, string) A comma-separated list of the following options:
@@ -70,7 +73,7 @@ you can request only the part of the cluster state that you need:
       Shows the `blocks` part of the response.
 
   `master_node`::
-      Shows the elected `master_node` part of the response.
+      Shows the `master_node` part of the response.
     
   `metadata`::
       Shows the `metadata` part of the response. If you supply a comma separated