Rollup merge of #124001 - ehuss:fix-unstable_features-docs, r=compiler-errors

Fix docs for unstable_features lint.

This fixes the `unstable_features` lint documentation (at https://doc.rust-lang.org/rustc/lints/listing/allowed-by-default.html#unstable-features) so that it correctly displays the output (instead of showing `{{produces}}`). The lint was undeprecated in https://github.com/rust-lang/rust/pull/118639, but this little part was missed when that happened.

src/tools/lint-docs/src/lib.rs

@@ -270,7 +270,6 @@ fn generate_output_example(&self, lint: &mut Lint) -> Result<(), Box<dyn Error>>
         if matches!(
             lint.name.as_str(),
             "unused_features" // broken lint
-            | "unstable_features" // deprecated
         ) {
             return Ok(());
         }