docs: improve tokio::io API documentation (#1815)

Adds method level documentation for `tokio::io`.

tokio/tests/buffered.rs

@@ -41,7 +41,7 @@ async fn echo_server() {
     let (mut a, _) = assert_ok!(srv.accept().await);
     let (mut b, _) = assert_ok!(srv.accept().await);
 
-    let n = assert_ok!(a.copy(&mut b).await);
+    let n = assert_ok!(io::copy(&mut a, &mut b).await);
 
     let (expected, t2) = t.join().unwrap();
     let actual = t2.join().unwrap();

tokio/tests/io_copy.rs

@@ -1,10 +1,9 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::io::{AsyncRead, AsyncReadExt};
+use tokio::io::{self, AsyncRead};
 use tokio_test::assert_ok;
 
-use std::io;
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
@@ -31,7 +30,7 @@ async fn copy() {
     let mut rd = Rd(true);
     let mut wr = Vec::new();
 
-    let n = assert_ok!(rd.copy(&mut wr).await);
+    let n = assert_ok!(io::copy(&mut rd, &mut wr).await);
     assert_eq!(n, 11);
     assert_eq!(wr, b"hello world");
 }

tokio/tests/tcp_echo.rs

@@ -35,7 +35,7 @@ async fn echo_server() {
     let (mut stream, _) = assert_ok!(srv.accept().await);
     let (mut rd, mut wr) = stream.split();
 
-    let n = assert_ok!(rd.copy(&mut wr).await);
+    let n = assert_ok!(io::copy(&mut rd, &mut wr).await);
     assert_eq!(n, (ITER * msg.len()) as u64);
 
     assert_ok!(rx.await);

tokio/tests/tcp_shutdown.rs

@@ -1,7 +1,7 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::io::AsyncWriteExt;
+use tokio::io::{self, AsyncWriteExt};
 use tokio::net::{TcpListener, TcpStream};
 use tokio::prelude::*;
 use tokio_test::assert_ok;
@@ -24,6 +24,6 @@ async fn shutdown() {
     let (mut stream, _) = assert_ok!(srv.accept().await);
     let (mut rd, mut wr) = stream.split();
 
-    let n = assert_ok!(rd.copy(&mut wr).await);
+    let n = assert_ok!(io::copy(&mut rd, &mut wr).await);
     assert_eq!(n, 0);
 }