docs: clarify when to make a continue request

[melissahenderson]

Hackathon feedback:

People struggled with understanding when to make the continue request once users had successfully accepted the payment. It would be helpful to document that they should be redirected to their app, which would have something like a “wait” keeping the process active until they completed the action or a polling scheme where the app retries call the call periodically until it succeeds, fails or gets a timeout. If you run a continuation request immediately, it’s going to fail because you need that explicit consent, so you have to wait for the user to click something; polling mechanism to see if its happened.