fix feature docs not showing up on docs.rs

[HyperCodec]

Probably should use the new #![cfg_attr(docsrs, feature(doc_cfg)]. Also add the following to Cargo.toml:

[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "docsrs"]

[HyperCodec]

note that the merged doc_cfg might not be updated on older crates and dependencies, so might have to fallback to older nightly + doc_auto_cfg instead.