Document the mobile guide toast option

Part of #16283

Author: jryans

docs/config.md

@@ -122,6 +122,9 @@ For a good example, see https://develop.element.io/config.json.
    This option is experimental and may be removed at any time without notice. It's
    also strongly advised not to set this option unless you're absolutly certain you
    know what you're doing.
+1. `mobileGuideToast`: Whether to show a toast a startup which nudges users on
+   iOS and Android towards the native mobile apps. The toast redirects to the
+   mobile guide if they accept. Defaults to false.
 
 Note that `index.html` also has an og:image meta tag that is set to an image
 hosted on riot.im. This is the image used if links to your copy of Element