docs: Multi-Page App note when using 'root' (#2939)

docs/guide/build.md

@@ -95,6 +95,8 @@ module.exports = {
 }
 ```
 
+If you specify a different root, remember that `__dirname` will still be the folder of your vite.config.js file when resolving the input paths. Therfore, you will need to add your `root` entry to the arguments for `resolve`. 
+
 ## Library Mode
 
 When you are developing a browser-oriented library, you are likely spending most of the time on a test/demo page that imports your actual library. With Vite, you can use your `index.html` for that purpose to get the smooth development experience.