[DOCS] Clarify compatibility and relation between high level REST client and core version (#26279)

Closes #26142

Author: javanna

docs/java-rest/high-level/usage.asciidoc

@@ -9,15 +9,27 @@ getting the artifact to using it in an application.
 The Java High Level REST Client requires Java 1.8 and depends on the Elasticsearch
 core project. The client version is the same as the Elasticsearch version that the
 client was developed for. It accepts the same request arguments as the `TransportClient`
-and returns the same response objects.
-
-The High Level Client is backwards compatible but can only communicate with Elasticsearch
-version 5.5 and onwards. The High Level Client is forward compatible as well, meaning that
-it supports communicating with a later version of Elasticsearch than the one it was developed
-for. It is recommended to upgrade the High Level Client when upgrading the Elasticsearch
-cluster to a new major version, as REST API breaking changes may cause unexpected results,
-and newly added APIs will only be supported by the newer version of the client. The client
-should be updated last, once all of the nodes in the cluster have been upgraded.
+and returns the same response objects. See the <<java-rest-high-level-migration>>
+if you need to migrate an application from `TransportClient` to the new REST client.
+
+The High Level Client is guaranteed to be able to communicate with any Elasticsearch
+node running on the same major version and greater or equal minor version. It
+doesn't need to be in the same minor version as the Elasticsearch nodes it
+communicates with, as it is forward compatible meaning that it supports
+communicating with later versions of Elasticsearch than the one it was developed for.
+
+The 6.0 client is able to communicate with any 6.x Elasticsearch node, while the 6.1
+client is for sure able to communicate with 6.1, 6.2 and any later 6.x version, but
+there may be incompatibility issues when communicating with a previous Elasticsearch
+node version, for instance between 6.1 and 6.0, in case the 6.1 client supports new
+request body fields for some APIs that are not known by the 6.0 node(s).
+
+It is recommended to upgrade the High Level Client when upgrading the Elasticsearch
+cluster to a new major version, as REST API breaking changes may cause unexpected
+results depending on the node that is hit by the request, and newly added APIs will
+only be supported by the newer version of the client. The client should always be
+updated last, once all of the nodes in the cluster have been upgraded to the new
+major version.
 
 [[java-rest-high-javadoc]]
 === Javadoc