fix(schema-first): add explanation about merging

Author: katainaka0503

packages/graphql/lib/federation/graphql-federation-definitions.factory.ts

@@ -37,6 +37,9 @@ export class GraphQLFederationDefinitionsFactory extends GraphQLDefinitionsFacto
       },
     ]);
 
+    // buildSubgraphSchema fills empty Query definition if there is `extend type Query` definition without original `type Query` definition.
+    // This causes duplicate IQuery interfaces.
+    // see: https://github.com/nestjs/graphql/issues/2344
     const mergedDefinition = mergeTypeDefs([printSubgraphSchema(schema)], {
       useSchemaDefinition: false,
       throwOnConflict: true,