Improve documentation for TcpStream::set_nonblocking

[mbrubeck]

These docs currently just refer to POSIX and Win32 APIs, without explaining what they do (or even linking to docs for the corresponding platform APIs):

https://doc.rust-lang.org/std/net/struct.TcpStream.html#method.set_nonblocking

I'd like to have more information here about how other TcpStream methods change their behavior after calling this, possibly with an example.

[frewsxcv]

I'd be happy to make the changes, though I actually don't know what the effects of using set_nonblocking are. If anyone knows, would be great if you could share

[tbu-]

This will result in read, write, recv and send operations not blocking, i.e. immediately returning from their calls. If the IO operation could be completed correctly, no further action is required. If it could not, EAGAIN or EWOULDBLOCK are returned on Unix platforms and WSAEWOULDBLOCK is returned on Windows. All of these are mapped to io::ErrorKind::WouldBlock, so on the Rust side of the things, you can simply check whether the return value of the IO operation has kind io::ErrorKind::WouldBlock, and in this case take the appropriate action.

[frewsxcv]

Opened a PR for TCP methods: #45227

I'll do another PR for the UDP instance of this in a follow-up PR when I have some more time.