Update the docs to clarify the changeOrigin option

This is misleadingly named in http-proxy

docs/config/server-options.md

@@ -90,7 +90,7 @@ Configure custom proxy rules for the dev server. Expects an object of `{ key: op
 
 Note that if you are using non-relative [`base`](/config/shared-options.md#base), you must prefix each key with that `base`.
 
-Extends [`http-proxy`](https://github.com/http-party/node-http-proxy#options). Additional options are [here](https://github.com/vitejs/vite/blob/main/packages/vite/src/node/server/middlewares/proxy.ts#L13).
+Extends [`http-proxy`](https://github.com/http-party/node-http-proxy#options). Additional options are [here](https://github.com/vitejs/vite/blob/main/packages/vite/src/node/server/middlewares/proxy.ts#L13). Note that the `changeOrigin` option changes host header to match the target URL, the origin header is always changed to match the target.
 
 In some cases, you might also want to configure the underlying dev server (e.g. to add custom middlewares to the internal [connect](https://github.com/senchalabs/connect) app). In order to do that, you need to write your own [plugin](/guide/using-plugins.html) and use [configureServer](/guide/api-plugin.html#configureserver) function.