Fix up event overview docs
Fix up event overview docs

Summary: Tweak event overview docs. Switch from MountSpec to LayoutSpec etc.

Reviewed By: mihaelao

Differential Revision: D4826202

fbshipit-source-id: 90657cf3b331d27cf117b9d682619bbe7a97a432

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho