docs: add devServer.inline in config reference (#6126) [skip ci]

closes #6031

docs/config/README.md

@@ -373,6 +373,16 @@ In v3 this means the opposite of `css.requireModuleExtension`.
   }
   ```
 
+### devServer.inline
+
+- Type: `boolean`
+- Default: `true`
+
+  Toggle between the dev-server's two different modes. See [devServer.inline](https://webpack.js.org/configuration/dev-server/#devserverinline) for more details. Note that:
+
+  - To use the `iframe mode` no additional configuration is needed. Just navigate the browser to `http://<host>:<port>/webpack-dev-server/<path>` to debug your app. A notification bar with messages will appear at the top of your app.
+  - To use the `inline mode`, just navigate to `http://<host>:<port>/<path>` to debug your app. The build messages will appear in the browser console.
+
 ### parallel
 
 - Type: `boolean | number`