doc: fix doc-cfg on io::duplex, io::copy_bidirectional, and task::unconstrained (#3773)

tokio/src/io/util/copy_bidirectional.rs

@@ -104,6 +104,7 @@ where
 /// # Return value
 ///
 /// Returns a tuple of bytes copied `a` to `b` and bytes copied `b` to `a`.
+#[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
 pub async fn copy_bidirectional<A, B>(a: &mut A, b: &mut B) -> Result<(u64, u64), std::io::Error>
 where
     A: AsyncRead + AsyncWrite + Unpin + ?Sized,

tokio/src/io/util/mem.rs

@@ -45,6 +45,7 @@ use std::{
 /// # }
 /// ```
 #[derive(Debug)]
+#[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
 pub struct DuplexStream {
     read: Arc<Mutex<Pipe>>,
     write: Arc<Mutex<Pipe>>,
@@ -80,6 +81,7 @@ struct Pipe {
 ///
 /// The `max_buf_size` argument is the maximum amount of bytes that can be
 /// written to a side before the write returns `Poll::Pending`.
+#[cfg_attr(docsrs, doc(cfg(feature = "io-util")))]
 pub fn duplex(max_buf_size: usize) -> (DuplexStream, DuplexStream) {
     let one = Arc::new(Mutex::new(Pipe::new(max_buf_size)));
     let two = Arc::new(Mutex::new(Pipe::new(max_buf_size)));

tokio/src/task/unconstrained.rs

@@ -5,6 +5,7 @@ use std::task::{Context, Poll};
 
 pin_project! {
     /// Future for the [`unconstrained`](unconstrained) method.
+    #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
     #[must_use = "Unconstrained does nothing unless polled"]
     pub struct Unconstrained<F> {
         #[pin]
@@ -38,6 +39,7 @@ where
 /// otherwise.
 ///
 /// See also the usage example in the [task module](index.html#unconstrained).
+#[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
 pub fn unconstrained<F>(inner: F) -> Unconstrained<F> {
     Unconstrained { inner }
 }