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`

Author: matthiaskrgr