[docs] Document breakpoint argument for withMobileDialog (#12521)

Add a sentence describing how one might choose their own breakpoint for their dialog to become full screen.

docs/src/pages/demos/dialogs/dialogs.md

@@ -67,7 +67,7 @@ For example, if your site prompts for potential subscribers to fill in their ema
 
 ## Responsive full-screen
 
-You may make a `Dialog` responsively full screen the dialog using `withMobileDialog`. By default, `withMobileDialog()(Dialog)` responsively full screens *at or below* the `sm` [screen size](/layout/basics).
+You may make a `Dialog` responsively full screen the dialog using `withMobileDialog`. By default, `withMobileDialog()(Dialog)` responsively full screens *at or below* the `sm` [screen size](/layout/basics). You can choose your own breakpoint for example `xs` by passing the `breakpoint` argument: `withMobileDialog({breakpoint: 'xs'})(Dialog)`.
 
 {{"demo": "pages/demos/dialogs/ResponsiveDialog.js"}}