process: document remote killing for Child (#2736)

* process: document remote killing for Child

Fixes: #2703

tokio/src/process/mod.rs

@@ -766,6 +766,32 @@ impl Child {
     /// Forces the child to exit.
     ///
     /// This is equivalent to sending a SIGKILL on unix platforms.
+    ///
+    /// If the child has to be killed remotely, it is possible to do it using
+    /// a combination of the select! macro and a oneshot channel. In the following
+    /// example, the child will run until completion unless a message is sent on
+    /// the oneshot channel. If that happens, the child is killed immediately
+    /// using the `.kill()` method.
+    ///
+    /// ```no_run
+    /// use tokio::process::Command;
+    /// use tokio::sync::oneshot::channel;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (send, recv) = channel::<()>();
+    ///     let mut child = Command::new("sleep").arg("1").spawn().unwrap();
+    ///     tokio::spawn(async move { send.send(()) });
+    ///     tokio::select! {
+    ///         _ = &mut child => {}
+    ///         _ = recv => {
+    ///             &mut child.kill();
+    ///             // NB: await the child here to avoid a zombie process on Unix platforms
+    ///             child.await.unwrap();
+    ///         }
+    ///     }
+    /// }
+
     pub fn kill(&mut self) -> io::Result<()> {
         self.child.kill()
     }