docs: io ReadHalf/WriteHalf docs are outdated

[taiki-e]

https://docs.rs/futures/latest/futures/io/struct.ReadHalf.html

The readable half of an object returned from AsyncRead::split.

https://docs.rs/futures/latest/futures/io/struct.WriteHalf.html

The writable half of an object returned from AsyncRead::split.

Both should be "... returned from AsyncReadExt::split"