`TODO: add notes link` in flax_basics guide

This very vague statement is in the flax_basics guide:

> "The last initialization will be handled differently. See these notes for more details (TODO: add notes link)."

I have no idea what that sentence might mean. What does `last initialization` refer to?

https://github.com/google/flax/blob/b967964377d032a31ea2f966c1d565ee0fecf2c5/docs/guides/flax_fundamentals/flax_basics.ipynb?short_path=e8e43f2#L670