MB-8685: added UI help text for flush

Change-Id: Ibe9bf160936972690bacfc2b4e6bfdcdedf699a3
Reviewed-on: http://review.couchbase.org/39427
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>