Add comment about bug in yields

This needs to be fixed somehow. The reconciler could know if you
are mounting this continuation into the same spot as before and
then clone it. However, if the continuation moves, this info is
lost. We'd try to unmount it in one place and mount the same fiber
in another place.

src/renderers/shared/fiber/ReactFiber.js

@@ -276,6 +276,11 @@ function createFiberFromElementType(type : mixed, key : null | string) {
     fiber.type = type;
   } else if (typeof type === 'object' && type !== null) {
     // Currently assumed to be a continuation and therefore is a fiber already.
+    // TODO: The yield system is currently broken for updates in some cases.
+    // The reified yield stores a fiber, but we don't know which fiber that is;
+    // the current or a workInProgress? When the continuation gets rendered here
+    // we don't know if we can reuse that fiber or if we need to clone it.
+    // There is probably a clever way to restructure this.
     fiber = type;
   } else {
     throw new Error('Unknown component type: ' + typeof type);