Mark all functions with their required CPU features

[Lokathor]

This is easy to do on a module-by-module basis, you just apply the attribute to each function in a module.

// example for "avx"
#[cfg_attr(docs_rs, doc(cfg(target_feature = "avx")))]

I just didn't start doing this at the beginning of crate development so someone has to go do this for previous work. With clever search and replace a person could probably update the entire crate in a matter of minutes.