warning attribute missing in diagnostics derive macro reference

[nicklimmm]

Previously, the warn_ attribute is used to add warning diagnostics. But it was changed to warning in this PR.

Some information is also outdated in both Diagnostic and Subdiagnostic derives, e.g. no warning reference in Subdiagnostic, the attribute is also applicable on structs.

Diagnostic derive documentation from nightly rustc (source of truth):

• rustc #[derive(Diagnostic)
• rustc #[derive(LintDiagnostic)]
• rustc #[derive(Subdiagnostic)]

What we have in the guide instead:

• #[derive(Diagnostic)]dev guide reference
• #[derive(Subdiagnostic)]dev guide reference

Updating the warning references in the guide can help new contributors to support the translation effort (issue tracker rust-lang/rust#100717)

[nicklimmm]

If this looks good, I'd like to update the docs accordingly