Rollup merge of #128033 - Nadrieril:explain-empty-wildcards, r=compiler-errors

Explain why we require `_` for empty patterns

This adds a note to the "non-exhaustive patterns" diagnostic to explain why we sometimes require extra `_` patterns on empty types. This is one of the two diagnostic improvements I wanted to do before [stabilizing `min_exhaustive_patterns`](https://github.com/rust-lang/rust/pull/122792).

r? ``@compiler-errors``