Document the new third callback arg of replicate()

lib/persisted-model.js

@@ -822,6 +822,8 @@ PersistedModel.currentCheckpoint = function(cb) {
  * @callback {Function} [callback] Callback function called with `(err, conflicts)` arguments.
  * @param {Error} err Error object; see [Error object](http://docs.strongloop.com/display/LB/Error+object).
  * @param {Conflict[]} conflicts A list of changes that could not be replicated due to conflicts.
+ * @param {Object] checkpoints The new checkpoints to use as the "since"
+ * argument for the next replication.
  */
 
 PersistedModel.replicate = function(since, targetModel, options, callback) {