Add vbuckets to feature descrtiption

Change-Id: I7ec7e71cd0068ed4a4065e9869adfce056f36adf
Reviewed-on: http://review.couchbase.org/29166
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>