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

[lazaruslarue]

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

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.