docs: add note about opening in a specific browser (#3976)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

docs/config/index.md

@@ -367,7 +367,7 @@ export default async ({ command, mode }) => {
 
 - **Type:** `boolean | string`
 
-  Automatically open the app in the browser on server start. When the value is a string, it will be used as the URL's pathname.
+  Automatically open the app in the browser on server start. When the value is a string, it will be used as the URL's pathname. If you want to open the server in a specific browser you like, you can set the env `process.env.BROWSER` (e.g. `firefox`). See [the `open` package](https://github.com/sindresorhus/open#app) for more details.
 
   **Example:**