Add Note to i18n-routing Docs for Subdomains for Locale Domains (#21387)

docs/advanced-features/i18n-routing.md

@@ -39,6 +39,7 @@ module.exports = {
     defaultLocale: 'en-US',
     // This is a list of locale domains and the default locale they
     // should handle (these are only required when setting up domain routing)
+    // Note: subdomains must be included in the domain value to be matched e.g. "fr.example.com".
     domains: [
       {
         domain: 'example.com',