Rollup merge of #106931 - Ezrashaw:docs-e0208, r=compiler-errors

Rollup merge of #106931 - Ezrashaw:docs-e0208, r=compiler-errors

document + UI test `E0208` and make its output more user-friendly

Cleans up `E0208`'s output a lot. It could actually be useful for someone learning about variance now. I also added a UI test for it in `tests/ui/error-codes/` and wrote some docs for it.

r? `@GuillaumeGomez` another error code, can't be bothered to find the issue :P. Obviously there's some compiler stuff, so you'll have to hand it off.

Part of https://github.com/rust-lang/rust/issues/61137.