Add doc note to _.forEach about array-like objects. [closes #339] [ci skip]

lodash.js

@@ -3434,6 +3434,10 @@
      * (value, index|key, collection). Callbacks may exit iteration early by
      * explicitly returning `false`.
      *
+     * Note: As with other "Collections" methods, objects with a `length` property
+     * are iterated like arrays. To avoid this behavior `_.forIn` or `_.forOwn`
+     * may be used for object iteration.
+     *
      * @static
      * @memberOf _
      * @alias each