Add private guard param doc to _.sortByOrder. [ci skip]

lodash.src.js

@@ -6975,6 +6975,7 @@
      * @param {Array|Object|string} collection The collection to iterate over.
      * @param {string[]} props The property names to sort by.
      * @param {boolean[]} orders The sort orders of `props`.
+     * @param- {Object} [guard] Enables use as a callback for functions like `_.reduce`.
      * @returns {Array} Returns the new sorted array.
      * @example
      *