docs: clarify purpose of/need for continue object in grant response

[melissahenderson]

Hackathon feedback:

I saw a few questions about why we have a continue object in all grant responses and which access token should be used. Max said that it’s a standard grant response format and the continue object is only needed for the outgoing payment grant. Implementations have the option to make other grants interactive if they wanted to as well.

See issue #542 for another example of the access token issue.