Expand the comment about ns_config:do_update_rec.

Change-Id: I3820ea2da129474461b1d781b2d369d3d6821db9
Reviewed-on: http://review.couchbase.org/39542
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>