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.

首先在 GraphQL 文档中找到 Union Types 部分，并将其中的指导与 Apollo 链接的 Unions and interfaces 示例进行比较。当文档解释 Union resolver 对 __resolveType 的要求，并展示或链接到一个有帮助的示例时，即可完成。

由索引模型根据 Issue 内容生成。