Update PropTypes for ReactElement & ReactNode

This updates the API to match our updated terminology, as well as bring us in line with our definition of a node. It does mean we're currently lying to people about renderables.

Reviewers: @sebmarkbage

Test Plan: npm run jest
Ah, I guess I misunderstood what we talked about, will restore.

I had considered splitting this so that we had a `node` check being strict and a separate `renderable` check maintaining the current behavior.