add win.showDevTools callback example

Author: ayushmanchhabra

docs/References/Window.md

@@ -297,7 +297,7 @@ Turn the window's native shadow on/off. Useful for frameless, transparent window
     The behavior of the function is changed since 0.13.0. Please see [Migration Notes from 0.12 to 0.13](../For Users/Migration/From 0.12 to 0.13.md).
 
 * `iframe` `{String} or {HTMLIFrameElement}` _Optional_ the id or the element of the `<iframe>` to be jailed on. By default, the DevTools is shown for entire window.
-* `callback(dev_win)` `{Function}` callback with the native window of the DevTools window.
+* `callback(dev_win, error)` `{Function}` callback with the native window of the DevTools window.
 
 Open the devtools to inspect the window.
 
@@ -309,6 +309,14 @@ This function returns a `Window` object via the callback. You can use any proper
 
 See also in [webview reference](webview Tag.md) on how to open DevTools for webview or open DevTools in a webview.
 
+```js
+win.showDevTools((dev_win, error) => {
+    if (error) {
+        // handle error
+    }
+});
+```
+
 ## win.closeDevTools()
 
 !!! note