chore: add build --watch documentation

docs/config/index.md

@@ -557,6 +557,13 @@ export default async ({ command, mode }) => {
 - **Default:** `500`
 
   Limit for chunk size warnings (in kbs).
+
+### build.watch
+
+- **Type:** [`WatcherOptions`](https://rollupjs.org/guide/en/#watch-options)`| null`
+- **Default:** `null`
+
+  Set to `{}` to enable rollup watcher. This is mostly used in cases that involve build-only plugins or integrations processes.
 
 ## Dep Optimization Options
 

docs/guide/build.md

@@ -46,6 +46,21 @@ module.exports = {
 
 For example, you can specify multiple Rollup outputs with plugins that are only applied during build.
 
+## Rebuild on files changes
+
+You can enable rollup watcher with `vite build --watch`. Or, you can directly adjust the underlying [`WatcherOptions`](https://rollupjs.org/guide/en/#watch-options) via `build.watch`:
+
+```js
+// vite.config.js
+module.exports = {
+  build: {
+    watch: {
+      // https://rollupjs.org/guide/en/#watch-options
+    }
+  }
+}
+```
+
 ## Multi-Page App
 
 Suppose you have the following source code structure: