Add note about notification when disabling autoPurge

Priya Rajagopal suggested that we should add note to the API doc when auto purge is disabled that 

When the autoPurge flag is disabled, the replicator will notify the registered DocumentReplication listeners with an "access removed" event when access to the document is revoked on the Sync Gateway. On receiving the event, the application may decide to manually purge the document. However, for performance reasons, any DocumentReplication listeners added to the replicator after the replicator is started will not receive the access removed events until the replicator is restarted or reconnected with Sync Gateway.