change in-memory database wording to include persistence

See second comment here: http://stackoverflow.com/questions/15224931/can-couchbase-server-work-with-ram-memory-only . User is confused that it is only in-memory.

Check out: http://www.couchbase.com/docs/couchbase-manual-2.0/couchbase-introduction-basics.html

Couchbase 2.0 documentation explicitly states that it's an in-memory database. From my experience the buckets all exist in RAM. You can set the size of every bucket to partition your RAM appropriately.