Java mention of Async libraries needs polish

Just above https://docs.couchbase.com/java-sdk/current/howtos/kv-operations.html#insert there’s a  NOTE: about async which makes no sense and goes nowhere.

Need to:

• move it to somewhere less jarring (we still want it, as we realise we are showing examples with the basic API, and want to indicate that there is something  "better")
• link it to https://docs.couchbase.com/java-sdk/current/howtos/concurrent-async-apis.html
• provide async variant of code (in howtos or devguide?)