Documentation doesn't have all available features

[Kerollmops]

The documentation of simd on http://doc.rs/simd doesn't have all available features. I know that cargo doc use the -C target_feature flag but http://doc.rs doesn't take that in account and compiles on its own arch.

For example I can't see the x86 avx f64x4 details.

The old @huonw documentation website already handle this problem 😸

[Kerollmops]

https://github.com/onur/docs.rs/pull/73