Add docs for baseSum and arraySum. [ci skip]

lodash.src.js

@@ -1699,6 +1699,13 @@
       return false;
     }
 
+    /**
+     * A specialized version of `_.sum` for arrays without support for iteratees.
+     *
+     * @private
+     * @param {Array} array The array to iterate over.
+     * @returns {number} Returns the sum.
+     */
     function arraySum(array) {
       var length = array.length,
           result = 0;
@@ -2757,6 +2764,15 @@
       });
     }
 
+    /**
+     * The base implementation of `_.sum` without support for callback shorthands
+     * and `this` binding.
+     *
+     * @private
+     * @param {Array|Object|string} collection The collection to iterate over.
+     * @param {Function} iteratee The function invoked per iteration.
+     * @returns {number} Returns the sum.
+     */
     function baseSum(collection, iteratee) {
       var result = 0;
       baseEach(collection, function(value, index, collection) {