async-std `TcpStream` impls `BufRead` w/ `AsyncWrite`

https://github.com/async-email/async-smtp/commit/eed4f0bfcc12e5cb5672073fbd83d56f3b81eec5

So wrapping TcpStream with BufReader should work ?

Even if I wrap it to BufReader then it wants AsyncWrite too complicating setting stuff like async-native-tls up ?

I could implement my own type with those implemented but probably doc neeeds to be adjusted - it's not as straightforward now in combination with the types it's usually used as the doc points to direct use with TcpStream.

```
45 | let mut transport = SmtpTransport::new(client, reader).await?;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `AsyncWrite` is not implemented for `std::io::BufReader>`
```

Repro: https://github.com/pinkforest/repro-async-std-async-smtp

For now I'm just using 0.8 as but got to refactor and port this stuff over off async-std probably

Start with the documentation or examples referenced by the direct TcpStream usage, then compare that guidance with the linked repro and the SmtpTransport::new trait error. Done means the documentation accurately explains the required AsyncWrite behavior when wrapping TcpStream or using TLS.

Written by the indexing model from the issue text.