Better description for EventAffordance `subscription` and `cancellation`

[relu91]

In EventAffordance section, we describe the different fields in table 7. During today's Scripting API call, we reviewed them and found out that the descriptions for subscription and cancellation could be improved.

In particular, both of them speak about a "Subscription", but it is not clear how they are related to the underlying WoT operation. I suggest the following:

• Subscription: Defines data that needs to be passed upon subscription with subscribeevent operation, e.g., filters or message format for setting up Webhooks.
• Cancellation: Defines any data that needs to be passed to unsubscribe to a subscription with unsubscribeevent operation, e.g., a specific message

[egekorkan]

There is also the table at https://w3c.github.io/wot-thing-description/#sec-op-data-schema-mapping but it is not linked to from enough places?