Docs: Improve the documentation around AsyncWriteExt::shutdown

[IvanUkhov]

For write_all, we have the following example:

use tokio::io::{self, AsyncWriteExt};
use tokio::fs::File;

#[tokio::main]
async fn main() -> io::Result<()> {
    let mut file = File::create("foo.txt").await?;

    file.write_all(b"some bytes").await?;
    Ok(())
}

It is correct if this is precisely the scenario at hand: we write, and we shutdown the runtime. However, one might get into non-deterministic troubles if one does not flush via shutdown before yielding the control:

#[tokio::main]
async fn main() -> io::Result<()> {
  write().await?;
  read().await?;
}

async fn write() -> io::Result<()> {
    let mut file = File::create("foo.txt").await?;
    file.write_all(b"some bytes").await?;
    Ok(())
}

async fn read() -> io::Result<()> {
    let mut file = File::open("foo.txt").await?;
    # …
    Ok(())
}

The read after the write might find the file incomplete, as the background thread for the write might still be around.