Add esdoc comments for .eachIndex() method

methods/eachIndex/index.js

@@ -1,3 +1,18 @@
+/**
+ *  Iterates the items of the array and passes their index to the callback.
+ *
+ *  @param {Array} array
+ *  @param {Function} callback - The function that will be passed the indeces
+ *      of each array item.
+ *
+ *  @return {Array} The original given array.
+ *
+ *  @example
+ *  eachIndex(['a', 'b', 'c'], i => console.log(i)); // prints: 0 1 2
+ *
+ *  @example
+ *  rbjs(['a', 'b', 'c']).eachIndex(i => console.log(i)); // prints: 0 1 2
+ */
 export default function eachIndex(array, callback) {
     if (typeof callback !== 'function') {
         throw new TypeError('Parameter "callback" must be a function.');