chore: add string param documentation

Author: martinmunillas

packages/use-intl/src/core/createFormatter.tsx

@@ -128,6 +128,7 @@ export default function createFormatter({
 
   function dateTime(
     /** If a number is supplied, this is interpreted as a UTC timestamp. */
+    /** If a string is supplied, this is interpreted as an ISO 8601 string. */
     value: Date | number | string,
     /** If a time zone is supplied, the `value` is converted to that time zone.
      * Otherwise the user time zone will be used. */