remove error code from `E0789`, add UI test/docs
`E0789` shouldn't have an error code, it's explicitly internal-only and is tiny in scope. (I wonder if we can tighten the standard for this in the RFC?) I also added a UI test and error docs (done like `E0208`, they are "no longer emitted").
r? `@GuillaumeGomez` (shouldn't need a compiler review, it's pretty minor)
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