[doc] mark parameter as optional

lib/isRequired.js

@@ -2,7 +2,7 @@
  * A function which can be used as a function parameter default which throws an error
  * if the parameter is undefined
  *
- * @param {string} varname - name of the variable for the output
+ * @param {string} [varname] - name of the variable for the output
  * @throws {Error}
  */
 function isRequired (varname) {