Add note to channel spec regarding channel subscription spec (#3132)

* Add note to channel spec regarding channel subscription spec

The spec.subscription fields shouldn't be set directly, rather
it should be set via the subscriptions themselves and then the status
should be propogated via the channel.

* Update wording based on feedback to clarify backing channel & channel

* linting suggestion change

Co-authored-by: Matt Moore <mattmoor@vmware.com>

* change CR to custom object

Co-authored-by: Adam Harwayne <harwayne@google.com>

* more linting

Co-authored-by: Matt Moore <mattmoor@vmware.com>

Co-authored-by: Matt Moore <mattmoor@vmware.com>
Co-authored-by: Adam Harwayne <harwayne@google.com>

docs/spec/channel.md

@@ -182,6 +182,10 @@ Each channel CRD MUST contain an array of subscribers:
 Each channel CRD MUST contain an array of subscribers:
 [`spec.subscribers`](https://github.com/knative/eventing/blob/master/pkg/apis/duck/v1beta1/subscribable_types.go)
 
+Note: The array of subscribers MUST NOT be set directly on the generic Channel
+custom object, but rather appended to the backing channel by the subscription
+itself.
+
 #### Status Requirements
 
 ##### v1alpha1 Status