Document async function as returning a promise.

app/scripts/lib/get-first-preferred-lang-code.js

@@ -8,7 +8,7 @@ const existingLocaleCodes = allLocales.map(locale => locale.code.toLowerCase().r
  * Returns a preferred language code, based on settings within the user's browser. If we have no translations for the
  * users preferred locales, 'en' is returned.
  *
- * @returns {string} A locale code, either one from the user's preferred list that we have a translation for, or 'en'
+ * @returns {Promise<string>} Promises a locale code, either one from the user's preferred list that we have a translation for, or 'en'
  *
  */
 async function getFirstPreferredLangCode () {