[docs] add reference to Class<T> from Class docs
[docs] add reference to Class<T> from Class docs

Summary: Our search for `Class` does not include `Class<T>` in the top results, so putting a reference to it on the class page will make it easier to find.

Reviewed By: dsainati1

Differential Revision: D19769883

fbshipit-source-id: 4e00fb843f9593db2bbd311d84c69a23d37df914

facebook/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,facebook/flow