Rollup merge of #128307 - ojeda:unescaped_backticks, r=GuillaumeGomez

Clean and enable `rustdoc::unescaped_backticks` for `core/alloc/std/test/proc_macro`

I am not sure if the lint is supposed to be "ready enough" (since it is `allow` by default), but it does catch a couple issues in `core` (`alloc`, `std`, `test` and `proc_macro` are already clean), so I propose making it `warn` in all the crates rendered in the website.

Cc: `@GuillaumeGomez`

library/proc_macro/src/lib.rs

@@ -37,6 +37,7 @@
 #![recursion_limit = "256"]
 #![allow(internal_features)]
 #![deny(ffi_unwind_calls)]
+#![warn(rustdoc::unescaped_backticks)]
 
 #[unstable(feature = "proc_macro_internals", issue = "27812")]
 #[doc(hidden)]