async_std::stream::Pending and async_std::stream::pending() are not documented as unstable items

[zjijz]

The stream::pending module is only exported for the "unstable" feature: https://github.com/async-rs/async-std/blob/4d9499c27751aa47bcd1c8f1cb0e56ee9a2e338b/src/stream/mod.rs#L328, but these items are not documented to require the feature in the docs like they are for other unstable functions/types: https://docs.rs/async-std/1.9.0/async_std/stream/index.html#traits.