Updated the description of ACCOUNT_STATUS_CHANGED, added a link to
the notifications use case guide, and added the
PRODUCT_TYPE_DEFINITION_CHANGE notification type.
Before, the description of every operation implied that the
notifications were being subscribed to. This didn't work for
delete operations, so the description of NotificationType
was generalized.
Removed the EXTERNAL_FULFILLMENT_SHIPMENT_STATUS_CHANGE notification
type. Added a disclaimer to the description for the FBA_OUTBOUND_SHIPMENT_STATUS
notification. Changes are in notifications.json