> there should be some way for a user to figure out if a variable
> *needs to be* in early-init.el or *can be* in init.el.

The set of variables which need to be set in early-init.el is exactly
the same as the set of variables which need to be set before
`package-initialize' is run, and hasn't changed as a result of this
patch. I do agree that adding explicit documentation of these
variables would be a good thing, though.