Document path stability of `f64::abs` in no std crates

[lopopolo]

I tried this code:

#![no_std]

pub fn xxx() {
    f64::abs(1.0);
}

In Rust older than 1.84.

I expected to see this happen: successful compilation since f64::abs is documented to be available since Rust 1.0.

Instead, this happened: compilation failure since f64::abs in no std programs wasn't stabilized until Rust 1.84. This isn't documented anywhere and I had to go issue diving on GitHub.

@RalfJung said the keywords here are documenting path stability vs item stability.

#131304 (comment)

Opening this issue to explore how to document the limited availability of items in no std programs.

In the absence of compiler infrastructure for this, free form text in the prose of the doc comment for the f64 primitive or its abs method would have solved this for me. Maybe in a # Caveats section?

[RalfJung]

Cc @rust-lang/rustdoc @rust-lang/libs-api

[GuillaumeGomez]

This is indeed pretty bad. I'll try to take a look whenever I have time if no one did until then.