Rollup merge of #139389 - mejrs:hidden, r=workingjubilee

make `Arguments::as_statically_known_str` doc(hidden)

Fixes `as_statically_known_str` being [visible](https://doc.rust-lang.org/nightly/std/fmt/struct.Arguments.html#method.as_statically_known_str) ([Rendered](https://github.com/user-attachments/assets/45482d9f-2ec5-4610-be9c-b231bd2850c6))

This snuck in with rust-lang/rust#138650, cc `@thaliaarchi`

This is also visible in the beta docs.

`@rustbot` label +beta-nominated

Author: Zalathar