Docs for Entities say __typename is required in reference but in practice it is not

The docs say:

> A representation **requires** only an explicit __typename definition and values for the entity's primary key fields.

and show the code:

```javascript
{
Review: {
product(review) {
return { __typename: "Product", upc: review.upc };
}
}
}
```

under the Resolving header: https://www.apollographql.com/docs/federation/entities/#resolving.

However, if you give the product field the type `Product` in the type defs, it will resolve with just upc, like so:

```javascript
{
Review: {
product(review) {
return { upc: review.upc };
}
}
}
```

Should the docs be updated to no longer require `__typename` in the reference? Something like:

> A representation requires only the values for the entity's primary key fields.

Start at the Resolving section of the federation entities documentation linked in the issue and compare its statement about __typename with the shown resolver examples. Verify the documented behavior against the Product-typed field case described, then update the wording if appropriate. Done means the reference accurately describes the required entity representation.

Written by the indexing model from the issue text.