[docs-infra] Leave TODO for warnOnce to reduce bundle size

Will help mui/mui-x#13911

packages/mui-docs/src/i18n/i18n.tsx

@@ -81,6 +81,7 @@ export function useSetUserLanguage() {
 
 const warnedOnce: Record<string, boolean> = {};
 
+// TODO, migrate to use warnOnce() helper
 const warn = (userLanguage: string, key: string, ignoreWarning: boolean) => {
   const fullKey = `${userLanguage}:${key}`;
   // No warnings in CI env