docs: add Node.js API update in migration guide (#3561)

Author: snitin315

migration-v4.md

@@ -545,3 +545,21 @@ There are a lot of other bug fixes.
   ```
 
   IE8 is not supported, sorry
+
+- Change in **Node.js API**:
+
+  - If you're using dev-server through the Node.js API, the options in devServer will be ignored. Pass the options as a first parameter instead:
+
+  v3:
+
+  ```js
+  new WebpackDevServer(compiler, {...})
+  ```
+
+  v4:
+
+  ```js
+  new WebpackDevServer({...}, compiler)
+  ```
+
+  - [See here](https://github.com/webpack/webpack-dev-server/tree/master/examples/api/simple) for an example of how to use `webpack-dev-server` through the Node.js API.