MB-100: added dev docs for ns_server's usage of checkpoints

Change-Id: If16c3ace2407f060e228cb212117af4c27d37e9a
Reviewed-on: http://review.couchbase.org/26445
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>