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
@sebmarkbage thoughts? "node" might be correct but it's also pretty confusing for consumers to see. "ReactNode" implies some sort of typing. Maybe "React node" (and then I would change the element one to "React element", I had changed it to "ReactElement")