warn of possible race condition in channel is_disconnected doc

diff --git a/library/std/src/sync/mpmc/mod.rs b/library/std/src/sync/mpmc/mod.rs
index 6249e8a..16ae8a8 100644
--- a/library/std/src/sync/mpmc/mod.rs
+++ b/library/std/src/sync/mpmc/mod.rs

@@ -626,6 +626,10 @@ pub fn same_channel(&self, other: &Sender<T>) -> bool {
 
     /// Returns `true` if the channel is disconnected.
     ///
+    /// Note that a return value of `false` does not guarantee the channel will
+    /// remain connected. The channel may be disconnected immediately after this method
+    /// returns, so a subsequent [`Sender::send`] may still fail with [`SendError`].
+    ///
     /// # Examples
     ///
     /// ```
@@ -1375,6 +1379,10 @@ pub fn iter(&self) -> Iter<'_, T> {
 
     /// Returns `true` if the channel is disconnected.
     ///
+    /// Note that a return value of `false` does not guarantee the channel will
+    /// remain connected. The channel may be disconnected immediately after this method
+    /// returns, so a subsequent [`Receiver::recv`] may still fail with [`RecvError`].
+    ///
     /// # Examples
     ///
     /// ```

diff --git a/library/std/src/sync/mpsc.rs b/library/std/src/sync/mpsc.rs
index 81bb584..2abf7bf 100644
--- a/library/std/src/sync/mpsc.rs
+++ b/library/std/src/sync/mpsc.rs

@@ -610,6 +610,10 @@ pub fn send(&self, t: T) -> Result<(), SendError<T>> {
 
     /// Returns `true` if the channel is disconnected.
     ///
+    /// Note that a return value of `false` does not guarantee the channel will
+    /// remain connected. The channel may be disconnected immediately after this method
+    /// returns, so a subsequent [`Sender::send`] may still fail with [`SendError`].
+    ///
     /// # Examples
     ///
     /// ```
@@ -1060,6 +1064,10 @@ pub fn try_iter(&self) -> TryIter<'_, T> {
 
     /// Returns `true` if the channel is disconnected.
     ///
+    /// Note that a return value of `false` does not guarantee the channel will
+    /// remain connected. The channel may be disconnected immediately after this method
+    /// returns, so a subsequent [`Receiver::recv`] may still fail with [`RecvError`].
+    ///
     /// # Examples
     ///
     /// ```