Add helper environment comment

Author: markdalgleish

packages/react-router-dev/vite/plugin.ts

@@ -915,6 +915,12 @@ export const reactRouterVitePlugin: ReactRouterVitePlugin = () => {
     };
   };
 
+  // We use a separate environment for loading the server manifest and inlined
+  // CSS during development. This is because "ssrLoadModule" isn't available if
+  // the "ssr" environment has been defined by another plugin (e.g.
+  // vite-plugin-cloudflare) as a Vite.DevEnvironment rather than a
+  // Vite.RunnableDevEnvironment:
+  // https://vite.dev/guide/api-environment-frameworks.html#runtime-agnostic-ssr
   const HELPER_ENVIRONMENT_NAME = "__react_router_helper__";
 
   const loadModule: LoadModule = (viteDevServer, url) => {