documentation should include `__resolveType` requirement for Union Type to return interface

There's no mention in the documentation that Union Types require a `__resolveType` field in the Resolver for the Union.

a good example of helpful information can be found at [Apollo's Documentation](https://www.apollographql.com/docs/graphql-tools/resolvers.html#Unions-and-interfaces)

> When you have a field in your schema that returns a union or interface type, you will need to specify an extra __resolveType field in your resolver map, which tells the GraphQL executor which type the result is, out of the available options.