[DOC] Add page for `DS.PromiseManyArray`

The following page will be rendered by `@class PromiseManyArray`(current master branch has no this page).
![2014-12-21 3 50 30](https://cloud.githubusercontent.com/assets/290782/5515902/bd232300-88c4-11e4-9049-6e0bd39bced3.png)