Document entity name/id + parentKey

In our entities we are not good at documenting what the `Model.id` and `Model.parentKey` is.

We should go through all entities and make sure this is documented for each `Kind`.
(This is important, as this is how an entity is looked up).

Sometimes we also duplicate properties from the `id` and `parentKey` because we can query them without an ancestor key, we should also document all cases where data is duplicated and an invariant must be manually maintained.