Fix up feature documentation with rustdoc

[dequbed]

Right now the features required for mechanisms to be available is written as plain text. Rustdoc has a feature to have the fact that a certain module needs a certain feature documented in a standart format.

It would be quite nice to switch to that as soon as that stabilizes.

[dequbed]

The rustdoc feature to document #[cfg(feature = "...")] is explained here: https://doc.rust-lang.org/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present

Until that is implemented it's probably best to somehow selectively enable use of it using #[cfg_attr(...

[duesee]

Hey @dequbed. Could you assign me this issue?