Protocol doesn't mention Subscription Response

[elf-pavlik]

https://solid.github.io/notifications/protocol#subscription

This section only describes a payload of Subscription Request. At the same time, various subscription types describe requirements on various Subscriptions Responses.

I think protocol should mention Subscription Response and shortly explain their role and mention that each subscription type will specify details of Subscription Response.

[elf-pavlik]

For example https://solid.github.io/notifications/linkeddatanotifications-subscription-2021#subscription-type

The inbox property is used in the body of the subscription request. The webid property is used in the body of the subscription response.