Document the named subject feature.

Closes #1566
> I've seen a few questions about this in IRC. I'd prefer to have a doc to point to.

Does what you have here answer any of those questions?  It looks like it just reiterates the error message we already provide.

> It always seemed a bit odd we prevent this but allow super with both let and an unnamed subject.

You can read up on the history of that in #804.