doc: add note about handler precedence and conflict warnings for includeRouter method

Author: arnabrahman

packages/event-handler/src/appsync-graphql/AppSyncGraphQLResolver.ts

@@ -188,6 +188,12 @@ class AppSyncGraphQLResolver extends Router {
    * All resolver handlers, batch resolver handlers, and exception handlers
    * from the included routers will be available in the current resolver.
    *
+   * **Note:** When multiple routers register handlers for the same type and field combination
+   * (e.g., both `userRouter` and `postRouter` define `Query.getPost`), the handler from the
+   * last included router takes precedence and will override earlier registrations.
+   * This behavior also applies to exception handlers registered for the same error class.
+   * A warning is logged to help you identify potential conflicts when handlers are overridden.
+   *
    * @example
    * ```ts
    * import { AppSyncGraphQLResolver, Router } from '@aws-lambda-powertools/event-handler/appsync-graphql';