cbstats key needs to discuss providing the vbucket id

http://developer.couchbase.com/documentation/server/4.5/cli/cbstats/cbstats-key.html discusses getting info about a key by passing the key and vbucket. That last part is actually very important as even though we do hash keys to vbuckets, the memcached binary protocol does not actually enforce and you can send any key to any vbucket.

People should be told how to find out the vbucket id of a particular key.