There is no hard guarantee that polling a second time will return
Poll::Ready, and this is particularly likely to break in the EOF case,
which is precisely where we don't need to do so at all.

Both tokio::io::BufReader and futures::io::BufReader always attempt to
read from the underlying reader when the buffer is empty, rather than
fusing EOF.

This makes them show up on the documentation page for the `stream`
module. It also makes the return types on the `TryStreamExt`
documentation page link to the adapter types.

… paired

… same stream

Also re-enable docs in CI.