MB-11924: [api.txt] added description of memcached settings api

Change-Id: If9041e5d6534ef01d77a503760d81472e42e8579
Reviewed-on: http://review.couchbase.org/42017
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>