Document the rustc_on_unimplemented attribute.

[mejrs]

Moves/improves rustc_on_unimplemented's docs from the dev guide to the compiler, then in the dev guide adds links to it and other relevant diagnostic/lint attributes

Rendered:

[rustbot]

The rustc-dev-guide subtree was changed. If your future PRs only touch the subtree, consider submitting them directly to rust-lang/rustc-dev-guide, which is where the document is primarily maintained (and has faster CI).

cc @BoxyUwU, @tshepang

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @TaKO8Ki

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.