Added note about adding subdomains of pantheonsite.io 03-platform-dom… (

#9306)

* Added note about adding subdomains of pantheonsite.io 03-platform-domains.md

Edge Routing recently put a limitation in place that prevents customers from adding custom domains in the pantheonsite.io namespace. https://getpantheon.atlassian.net/browse/EDRT-4140

While it was never recommended or prescribed, there are a handful of customers that were engaging in this practice who are now confused about the limitation.

* use backticks for url

---------

Co-authored-by: Chris Reynolds <chris.reynolds@pantheon.io>

source/content/guides/domains/03-platform-domains.md

@@ -24,7 +24,11 @@ This section provides information Pantheon platform domains.
 
 All platform domains are available over HTTPS. Redirecting to HTTPS during development and testing is a recommended best practice to ensure you are ready to go live with HTTPS. Refer to [Redirect to HTTPS](/guides/redirect/#redirect-to-https) for more information.
 
+<Alert title="Note" type="info">
 
+Platform domains are provisioned automatically and are non-configurable. Attempting to add variants of a platform domain (e.g. `foo.site-name.pantheonsite.io`) as a [custom domain](https://docs.pantheon.io/guides/domains/custom-domains) will result in an error. 
+
+</Alert>
 
 ### robots.txt