Link to Eventing Data Plane Contract/SPEC

[embano1]

Describe the change you'd like to see
Speaking to many function authors (i.e. Knative Service with Eventing), especially newcomers, most of them are not aware of the underlying data plane contract, e.g. used by brokers. This can lead to issues where retry and error behavior by a function (Kservice) is incorrectly assumed/handled.

Examples SPEC says to retry on 404 which is typically not what users expect (404s are usually not retriable and the SPEC bends the rules here IMHO a bit for a particular use case).

Proposal: add a section to the docs describing the impact of the data plane contract to Knative function authors so at least they are aware of the behavior. The only issue is that not all implementations might strictly follow the SPEC, so this should be considered best effort/best practice for now until we are sure that all implementations strictly follow the SPEC.

Additional context
Add any other context or screenshots about the feature request here.

[abrennan89]

@knative-sandbox/func-wg-leads @knative/eventing-wg-leads can ya'll provide some information here about what needs to be documented?

[pierDipi]

This is the spec version of the content to document https://github.com/knative/specs/blob/main/specs/eventing/data-plane.md#event-acknowledgement-and-delivery-retry

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