No documentation about f32_runtime without std

[elichai]

Hi,
It seems like all the functions under f32_runtime don't work without std (https://doc.rust-lang.org/src/std/f32.rs.html#29-994)
I tried looking for documentation about this and didn't find anything concrete, and since the core docs don't have documentation for the primitives and only the std ones do it's hard to figure this out.
Some examples:
https://play.rust-lang.org/?gist=5aa47f506cd792cf58fbcd9f0219ef2b

According to: #39473
It sounds like some of it may be on purpose? if so it's too bad but at least should be documented well

[elichai]

This can be a real problem.

i.e. if you need to calc 2^-36 you can do (-36f32).exp2().
Without access to this you might think you can just do:
let a: f32 = 1.0 / 2u32.pow(36) as f32
But this will overflow.

Is there any solution to this?

[varkor]

The tracking issue for this is #50145. @Lokathor's been making good progress recently towards addressing this problem in #63455. (So it's possible that soon enough, extra documentation won't be necessary.)