MB-11939: mention that fullEviction policy doesn't retain keys

Change-Id: Ib2289aa776612ae9ab118a96667f4691ae54b22f
Reviewed-on: http://review.couchbase.org/40576
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>