refactor: improve visibility of TXT affecting apex domain (platformsh…

…#4285)

* refactor: improve visibility of TXT affecting apex domain

* Update subdomains.md

---------

Co-authored-by: Chad Carlson <[email protected]>

sites/platform/src/domains/steps/subdomains.md

@@ -36,9 +36,15 @@ This ensures no other users can possibly add a subdomain of your domain to their
 Even if you don’t remove the record, your DNS records should prevent others from using a subdomain
 as long as you don’t use wildcards records pointing at {{% vendor/name %}}.
 
-However, if you don't remove the `TXT` record, restrictions apply on the apex domain.
+{{< note theme="info" title="Apex domain cannot be re-added" >}}
+
+If you don't remove the `TXT` record, restrictions apply on the apex domain.
 For example, you can't add the apex domain to another project until you remove the `TXT` record.
 
+You can see an error like `YOUR_APEX_DOMAIN is a root domain`.
+
+{{< /note >}}
+
 ## Bypass locked domains
 
 In certain cases (such as if your domain was added manually by {{% vendor/name %}} support),