corrected api description of delta recovery

Change-Id: I198b44a06e25847126ae75aa0835fddfe3bd54bd
Reviewed-on: http://review.couchbase.org/38889
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>