Update custom-domains.mdx

apps/docs/pages/guides/platform/custom-domains.mdx

@@ -145,6 +145,8 @@ As with the final activation stage of the process for setting up a vanity subdom
 
 Edge functions do not honor the custom domain or the vanity subdomain setting and they still have to be invoked via the `foobarbaz.supabase.co` domain.
 
+A Supabase project can-at this time-use either a Custom Domain, or a Vanity Subdomain, but not both.
+
 export const Page = ({ children }) => <Layout meta={meta} children={children} />
 
 export default Page