tokio: make all windows docs visible in unix builds

[satakuma]

This PR makes all Windows-specific documentation visible in Unix builds similarly to the docs for named pipes.

Items which should be now visible:

• tokio::fs::symlink_dir
• tokio::fs::symlink_file
• tokio::fs::OpenOptions methods (custom_flags, access_mode, share_mode, attributes, security_qos_flags)
• {AsRawHandle, FromRawHandle} impls for tokio::fs::File
• AsRawHandle impl for tokio::io::{Stdin, Stdout, Stderr}
• AsRawSocket impl for tokio::net::{TcpListener, TcpStream, UdpSocket}
• {AsRawSocket, FromRawSocket, IntoRawSocket} for tokio::net::TcpSocket
• AsRawHandle impl for tokio::process::{ChildStdin, ChildStdout, ChildStderr}
• tokio::process::Command::creation_flags
• tokio::process::Child::raw_handle

View rendered docs

Resolves #5499

[satakuma]

Now I see that this will merge conflict with #5514, I can wait until that PR is merged first.

[satakuma]

Updated to include AsHandle/AsSocket implementations as well.