Java 3.0: example missing xattrs section in the subdoc api

the xattrs section in the subdoc API is stating the following:

Extended Attributes

Extended Attributes (also known as XATTRs), built upon the Sub-Document API, allow developers to define application-specific metadata that will only be visible to those applications that request it or attempt to modify it. This might be, for example, meta-data specific to a programming framework that should be hidden by default from other frameworks or libraries, or possibly from other versions of the same framework. They are not intended for use in general applications, and data stored there cannot be accessed easily by some Couchbase services, such as Search.

However, there is no example of how to use it.