MB-100: added dev docs for ns_server's usage of checkpoints
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 <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

daverigby/ns_server,ceejatec/ns_server,ceejatec/ns_server,daverigby/ns_server,ceejatec/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,membase/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,vzasade/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,daverigby/ns_server,mschoch/ns_server,nimishzynga/ns_server,mschoch/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,membase/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server