Update documentation for CollectionProxy

[marcandre]

CollectionProxy doesn't undefine class and other basic methods (halleluia) since c86a32d and this commit changes the doc to reflect this.

[carlosantoniodasilva]

Thanks.

[KensoDev]

@marcandre b.t.w, for documentation you can use: https://github.com/lifo/docrails

It's a public access repo, you can just push (only documentation changes), and those will get merged into rails every few days.

[marcandre]

@KensoDev Thanks, will do next time.