[DOC beta] Make `Ember.computed.collect` visible in docs

**before**

![:shipit:](https://f.cloud.github.com/assets/341877/1823328/ae298bc2-7179-11e3-8681-17791cb8c402.png)

**after**

![:+1:](https://f.cloud.github.com/assets/341877/1823329/b2703cd0-7179-11e3-891c-ba35ed4f1b5a.png)

![:trollface:](https://f.cloud.github.com/assets/341877/1823330/b57fef24-7179-11e3-8405-92f7c4efad2e.png)