Clarification of the protocol and sink properties in the subscription data model.

Author: tlohmar

documentation/API-design-guidelines.md

@@ -1759,8 +1759,8 @@ The following table provides `/subscriptions` attributes
 
 | name           | type               | attribute description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | cardinality                  |
 |----------------|--------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------| 
-| protocol       | string             | Identifier of a delivery protocol. **Only** `HTTP` **is allowed for now**.                                                                                                                                                                                                                                                                                                                                                                                                                                                               | Mandatory                    |
-| sink           | string             | The address to which events shall be delivered, using the HTTP protocol.                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | mandatory                    |
+| protocol       | string             | Identifier of a delivery protocol for the event notifications. The values follow the definitions of the [CloudEvent specification](https://github.com/cloudevents/spec/blob/main/subscriptions/spec.md#protocol). **Only** `HTTP` **is allowed for now**.                                                                                                                                                                                                                                                                                                                                                                                                                                                               | mandatory                    |
+| sink           | string             | The URL, to which event notifications shall be sent. The URI-scheme shall be set according to the definition of the `protocol` value, e.g. the URI-scheme is `https` when `HTTP`is the value of the `protocol` property. The [security considerations](#correlation-management) should be followed.                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | mandatory                    |
 | sinkCredential | object             | Sink credential provides authorization information necessary to enable delivery of events to a target. In order to be updated in future this object is polymorphic. See detail below. To protect the notification endpoint providing sinkCredential is RECOMMENDED. <br> The sinkCredential must **not** be present in `POST` and `GET` responses.                                                                                                                                                                                                                                                                    | optional                     |
 | types          | string             | Type of event subscribed. This attribute **must** be present in the `POST` request. It is required by API project to provide an enum for this attribute. `type` must follow the format: `org.camaraproject.<api-name>.<api-version>.<event-name>` with the `api-version` with letter `v` and the major version like  ``org.camaraproject.device-roaming-subscriptions.v1.roaming-status`` - Note: An array of types could be passed **but as of now only one value MUST passed**. Use of multiple value will be open later at API level. | mandatory                    |
 | config         | object             | Implementation-specific configuration parameters needed by the subscription manager for acquiring events. In CAMARA we have predefined attributes like ``subscriptionExpireTime``, ``subscriptionMaxEvents`` or ``initialEvent``. See detail below.                                                                                                                                                                                                                                                                                      | mandatory                    |