[shell] Add references to VsyncWaiter docs (flutter#44607)

Point users in the direction of VsyncWaiterAndroid, VsyncWaiterEmbedder.

No new tests since this patch introduces no semantic changes.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

shell/common/vsync_waiter.h

@@ -18,6 +18,9 @@ namespace flutter {
 
 /// Abstract Base Class that represents a platform specific mechanism for
 /// getting callbacks when a vsync event happens.
+///
+/// @see VsyncWaiterAndroid
+/// @see VsyncWaiterEmbedder
 class VsyncWaiter : public std::enable_shared_from_this<VsyncWaiter> {
  public:
   using Callback = std::function<void(std::unique_ptr<FrameTimingsRecorder>)>;