Add missing docs

[GuillaumeGomez]

Needed by rust-lang/rust#56407

[gnzlbg]

Thanks!

[GuillaumeGomez]

Question: after updating to last version, it prevents rustc to compile because of:

error[E0658]: the target feature `avx512f` is currently unstable (see issue #44839)e
  --> src/libcore/../stdsimd/coresimd/x86/avx512f.rs:13:18
   |
13 | #[target_feature(enable = "avx512f")]
   |                  ^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(avx512_target_feature)] to the crate attributes to enable

Am I supposed to enable the feature as asked or is it a known issue?

[gnzlbg]

That feature is indeed unstable, and I think we have started using it since the last time that stdsimd was updated in the rust-lang/rust repo, so it is likely that libcore has to be updated to enable the feature for x86 and x86_64 targets.