[DOCS] Added note to high level client docs on version

The alpha2 docs is built out of master which may make users think that the high level client was already released as part of alpha2 which it was not. This note should clarify that the client will be released with 6.0.0-beta1

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

@@ -3,6 +3,8 @@
 
 [partintro]
 --
+added[6.0.0-beta1]
+
 The Java High Level REST Client works on top of the Java Low Level REST client.
 Its main goal is to expose API specific methods, that accept request objects as
 an argument and return response objects, so that request marshalling and