MB-100: mention that old stats are lost on upgrade from mnesia

Change-Id: Ib9f49244119f04b48ff0b90ea5aa88f01c506ba1
Reviewed-on: http://review.couchbase.org/25424
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>