Use #[doc(cfg(...))] #295

A lot of our API is behind #[cfg]. Currently, that fact is only sometimes indicated in the documentation, which makes it hard to figure out, but as it turns out rustdoc has a feature for that: #[doc(cfg(<...>))].

This can be used to decorate types and method with information about their availability. An example of this can be seen in the async-std docs, which marks some APIs as unstable.