Document rules for root procs, give better error messages

The documentation doesn't really touch on what is required of a root proc, and errors are sometimes mysterious. It makes sense that the root proc can't have recurrent state or be parametric, but it might not be so clear from the error messages what's going on.

735d2d99b56d8626ce1e8a81b3ddd183458dfdad shows some examples that I think should be discussed in documentation and perhaps have better error messages specific to being a root proc.