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

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.

Start with src/stream/mod.rs at the stream::pending module export, then compare its documentation with the other unstable functions and types shown in the docs.rs stream index. Update the pending module and item documentation so the unstable feature requirement is visible, then verify the generated documentation reflects that requirement.

Written by the indexing model from the issue text.