`cargo doc` fails if they run it with nightly Rust

[A1-Triard]

STR: run cargo +nightly doc. ER: no errors. AR:

error[E0658]: custom classes in code blocks are unstable
  --> src/identities.rs:4:1
   |
4  | / /// Defines an additive identity element for `Self`.
5  | | ///
6  | | /// # Laws
7  | | ///
...  |
10 | | /// 0 + a = a       ∀ a ∈ Self
11 | | /// ```
   | |_______^
   |
   = note: see issue #79483 <https://github.com/rust-lang/rust/issues/79483> for more information
   = help: add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
   = note: found these custom classes: class=text

error[E0658]: custom classes in code blocks are unstable
  --> src/identities.rs:80:1
   |
80 | / /// Defines a multiplicative identity element for `Self`.
81 | | ///
82 | | /// # Laws
83 | | ///
...  |
86 | | /// 1 * a = a       ∀ a ∈ Self
87 | | /// ```
   | |_______^
   |
   = note: see issue #79483 <https://github.com/rust-lang/rust/issues/79483> for more information
   = help: add `#![feature(custom_code_classes_in_docs)]` to the crate attributes to enable
   = note: found these custom classes: class=text

[cuviper]

I think this is a rustdoc regression, as there was no error from nightly a week ago.

[robamu]

I think this might be due to https://github.com/rust-lang/rust/pull/110800