docs: update library mode info (#9102)

docs/guide/build.md

@@ -163,27 +163,32 @@ Running `vite build` with this config uses a Rollup preset that is oriented towa
 ```
 $ vite build
 building for production...
-[write] my-lib.mjs 0.08kb, brotli: 0.07kb
-[write] my-lib.umd.js 0.30kb, brotli: 0.16kb
+dist/my-lib.js 0.08 KiB / gzip: 0.07 KiB
+dist/my-lib.umd.cjs 0.30 KiB / gzip: 0.16 KiB
 ```
 
 Recommended `package.json` for your lib:
 
 ```json
 {
  "name": "my-lib",
+ "type": "module",
  "files": ["dist"],
- "main": "./dist/my-lib.umd.js",
- "module": "./dist/my-lib.mjs",
+ "main": "./dist/my-lib.umd.cjs",
+ "module": "./dist/my-lib.js",
  "exports": {
  ".": {
- "import": "./dist/my-lib.mjs",
- "require": "./dist/my-lib.umd.js"
+ "import": "./dist/my-lib.js",
+ "require": "./dist/my-lib.umd.cjs"
  }
  }
 }
 ```
 
+::: tip Note
+If the `package.json` does not contain `"type": "module"`, Vite will generate different file extensions for Node.js compatibility. `.js` will become `.mjs` and `.cjs` will become `.js`.
+:::
+
 ## Advanced Base Options
 
 ::: warning