JCBC-558: Javadocs for bucket and async bucket.

Change-Id: Icb8ae300d9141069ff652a149b19eada59dccded
Reviewed-on: http://review.couchbase.org/41492
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>