Publish unstable documentation to docs.rs

[Mark-Simulacrum]

Security issue notifications

If you discover a potential security issue in s2n-quic we ask that you notify
AWS Security via our vulnerability reporting page. Please do not create a public github issue.

Problem:

Experimenting with as-yet unstable features (e.g., datagrams) is relatively annoying, because all of the API surface area isn't visible on docs.rs, which means that you need to build documentation locally to know which traits exist where, etc.

Solution:

tokio does this via docs.rs metadata (https://github.com/tokio-rs/tokio/blob/master/tokio/Cargo.toml#L170), and uses rustdoc support to have the instability show up on various types/methods (e.g., https://docs.rs/tokio/latest/tokio/runtime/struct.RuntimeMetrics.html has a note that it is "Available on tokio_unstable and crate feature rt only.").

A similar practice could likely be adopted for s2n-quic.

[maddeleine]

Could you give us an example of an API you're not able to reach because it's hidden with unstable? I'm probably missing something, but I do see unstable datagram APIs on docs.rs.

For example, I can see the poll_send_datagram() API on the public docs site.

[camshaft]

That's linked in core. We need to expose it in the s2n-quic crate itself