Rollup merge of rust-lang#155039 - WaffleLapkin:hard_unstable, r=JonathanBrouwer

minor follow up to removing soft mode `#[unstable]`

Follow up to rust-lang#153622
r? JonathanBrouwer

Author: JonathanBrouwer

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

@@ -332,7 +332,6 @@ impl<'a> LintExtractor<'a> {
         if matches!(
             lint.name.as_str(),
             "unused_features" // broken lint
-            | "soft_unstable" // cannot have a stable example
         ) {
             return Ok(());
         }