MB-12564: added api.txt description for service-ful node addition

Change-Id: I80052a6c5b485cd3ab565bc1c489e60c530a082c
Reviewed-on: http://review.couchbase.org/43145
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>