message: docs(migrate-9): add section about build order change
diff: @@ -16,3 +16,7 @@ Mongoose version `6.0.1` (and possibly higher) is now supported, for mongoose sp
  ## Class-transformer transform of "ObjectId" became broken
  Since mongodb 5.0 (mongoose 6.0) the `value` in `@Transform` is not equals to `obj[key]` anymore, see [the updated Class-Transformer Guide](./advanced/class-transformer.md#implementation) and [the Issue about this](https://github.com/typestack/class-transformer/issues/879)
  +
  +## Build order for Class-to-Schema changed
  +
  +The build order for Class-to-Schema (`buildSchema`) changed from "top-down" to "bottom-up"
  