Rollup merge of rust-lang#146314 - cyrgani:hide-fmt-args-nl, r=GuillaumeGomez

mark `format_args_nl!` as `#[doc(hidden)]`

The `#[unstable]` attribute of the macro already says:

> `format_args_nl` is only for internal language use and is subject to change

It does seem plausible to hide it from the `std` docs accordingly.

The PR also removes the single usage of the macro outside of `std` as it does not seem like the macro is actually needed there.

Author: Zalathar

core/src/macros/mod.rs

@@ -1017,6 +1017,7 @@ pub(crate) mod builtin {
     )]
     #[allow_internal_unstable(fmt_internals)]
     #[rustc_builtin_macro]
+    #[doc(hidden)]
     #[macro_export]
     macro_rules! format_args_nl {
         ($fmt:expr) => {{ /* compiler built-in */ }};