Improve the docs about "modules" when initialize ApolloServer

[mrdulin]

Don't find any doc about "modules" config. Code here:
server.ts:

import { ApolloServer } from 'apollo-server';
import { Post, Tag } from './modules';

const server = new ApolloServer({
  modules: [Post, Tag],
  playground: true,
  introspection: true,
});

server.listen().then(({ url }) => {
  console.log(`apollo server is listening on ${url}`);
});

types.ts:

export interface Config extends BaseConfig {
  modules?: GraphQLSchemaModule[];
  // ...
}

Please add a doc and usage for it. I create a minimal example, but it doesn't work. Got error:

Type "Query" was defined more than once.

repo: https://github.com/mrdulin/apollo-graphql-tutorial/tree/master/src/initialize-with-modules

[justinmchase]

You have to extend a type that appears in multiple modules.

extend type Query {
  posts: [Post]!
}

extend type Query {
  tags: [Tag]!
}

[glasser]

Duplicate of #3642