document design principle around nullability

[baywet]

We keep getting questions about nullability when we project the client.
We should document the facts that:

• we project a nullable return type for the root of the response, even if the description doesn't say it's nullable. This is so the client can survive change in service behaviour without being refreshed. (e.g. the service starts returning 204)
• we project properties on models as nullable. This is to account for services that allow selection/projection via query parameters where the returned payload might omit described properties.
• we project request bodies as nullable or not depending on the required property in the description