Add yui docs for DS.belongsTo and DS.hasMany

### Before
![screen shot 2013-12-19 at 12 41 33 pm](https://f.cloud.github.com/assets/54056/1784607/62a87976-68d5-11e3-8e8e-702fb6c40d84.png)



### After
![screen shot 2013-12-19 at 12 39 01 pm](https://f.cloud.github.com/assets/54056/1784606/61608b26-68d5-11e3-8cad-6c2cb34bca95.png)