docs: update schema section to fix broken link

Author: glasser

docs/source/api/apollo-server.mdx

@@ -143,7 +143,12 @@ The default value is `true`, **unless** the `NODE_ENV` environment variable is s
 
 An executable GraphQL schema. You usually don't need to provide this value, because Apollo Server generates it from [`typeDefs`](#typedefs) and [`resolvers`](#resolvers).
 
-This field is most commonly used with [Apollo Federation](https://www.apollographql.com/docs/federation/implementing-services/#generating-a-federated-schema), which uses a special `buildFederatedSchema` function to generate its schema.
+This field is helpful if:
+
+- You are [building a subgraph for Apollo Federation](https://www.apollographql.com/docs/federation/building-supergraphs/subgraphs-apollo-server), which uses the `buildSubgraphSchema` function to generate its schema
+- You are using a function like `makeExecutableSchema` from [`graphql-tools`](https://www.graphql-tools.com/docs/generate-schema) to create your schema
+- You are using a library that takes a code-first approach (i.e., instead of a schema-first approach) to generate a schema
+
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