Make some lint doctests compatible with --stage=0

compiler/rustc_lint/src/if_let_rescope.rs

@@ -24,7 +24,7 @@
     /// ### Example
     ///
     /// ```rust,edition2021
-    /// #![feature(if_let_rescope)]
+    /// #![cfg_attr(not(bootstrap), feature(if_let_rescope))] // Simplify this in bootstrap bump.
     /// #![warn(if_let_rescope)]
     /// #![allow(unused_variables)]
     ///

compiler/rustc_lint_defs/src/builtin.rs

@@ -1871,6 +1871,7 @@
     /// ### Example
     ///
     /// ```rust,compile_fail
+    /// # #[cfg_attr(bootstrap)] compile_error!(); // Remove this in bootstrap bump.
     /// #![deny(elided_named_lifetimes)]
     /// struct Foo;
     /// impl Foo {