The changed field in the update is 'actions'.
The "realtime updates" section of the payment object documentation (https://developers.facebook.com/docs/reference/api/payment/) gives refunds and chargebacks as examples of cases the field can be update after completion.
Also, the "Handling Disputes and Refunds" page you quote, while not directly mentioning anything about the details of the realtime update, does have examples about the Graph API returns for transactions after a Facebook-initiated refund or chargeback has happened, in both of those examples, the refund/chargeback information has been added as a new entry to the actions field.
I do not believe there is a way to test disputes or Facebook-initiated refunds/chargebacks except with real money payments, but you can refund a non-disputed test payment yourself and that results in a realtime update that should be identical with one initiated by Facebook.