Add a few notes on Cancellable to the LLRC and HLRC docs. (#45912)

Add a section to both the low level and high level client documentation on asynchronous usage and `Cancellable` added for #44802 

Co-Authored-By: Lee Hinman <dakrone@users.noreply.github.com>

docs/java-rest/high-level/getting-started.asciidoc

@@ -154,3 +154,23 @@ executes the request. For example, this is the place where you'd specify a
 `NodeSelector` to control which node receives the request. See the
 <<java-rest-low-usage-request-options,low level client documentation>> for
 more examples of customizing the options.
+=== Asynchronous usage
+
+All of the the methods across the different clients exist in a traditional synchronous and 
+asynchronous variant. The difference is that the asynchronous ones use asynchronous requests 
+in the REST Low Level Client. This is useful if you are doing multiple requests or are using e.g.
+rx java, Kotlin co-routines, or similar frameworks.
+
+The asynchronous methods are recognizable by the fact that they have the word "Async" in their name 
+and return a `Cancellable` instance. The asynchronous methods accept the same request object 
+as the synchronous variant and accept a generic `ActionListener<T>` where `T` is the return 
+type of the synchronous method. 
+
+All asynchronous methods return a `Cancellable` object with a `cancel` method that you may call 
+in case you want to abort the request. Cancelling
+no longer needed requests is a good way to avoid putting unnecessary 
+load on Elasticsearch.
+
+Using the `Cancellable` instance is optional and you can safely ignore this if you have 
+no need for this. A use case for this would be using this with e.g. Kotlin's `suspendCancellableCoRoutine`. 
+

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

@@ -338,6 +338,12 @@ the underlying http client. On the server side, this does not automatically
 translate to the execution of that request being cancelled, which needs to be
 specifically implemented in the API itself.
 
+The use of the `Cancellable` instance is optional and you can safely ignore this
+if you don't need it. A typical usecase for this would be using this together with 
+frameworks like Rx Java or the Kotlin's `suspendCancellableCoRoutine`. Cancelling
+no longer needed requests is a good way to avoid putting unnecessary 
+load on Elasticsearch.
+
 ["source","java",subs="attributes,callouts,macros"]
 --------------------------------------------------
 include-tagged::{doc-tests}/RestClientDocumentation.java[rest-client-async-cancel]