docs: clarify when import.meta.env.PROD is true (#14124)

guide/env-and-mode.md

@@ -8,7 +8,7 @@ Vite exposes env variables on the special **`import.meta.env`** object. Some bui
 
 - **`import.meta.env.BASE_URL`**: {string} the base url the app is being served from. This is determined by the [`base` config option](/config/shared-options.md#base).
 
-- **`import.meta.env.PROD`**: {boolean} whether the app is running in production.
+- **`import.meta.env.PROD`**: {boolean} whether the app is running in production (running the dev server with `NODE_ENV='production'` or running an app built with `NODE_ENV='production'`).
 
 - **`import.meta.env.DEV`**: {boolean} whether the app is running in development (always the opposite of `import.meta.env.PROD`)