channel/recv: improving function docs and code example

At the moment it's not clear when and why recv returns Option<T>,
instead of just T. This changed comment makes it clear that None will
only be returned once no data will ever be sent again (i.e. after all
senders are gone).

src/sync/channel.rs

@@ -346,8 +346,9 @@ pub struct Receiver<T> {
 impl<T> Receiver<T> {
     /// Receives a message from the channel.
     ///
-    /// If the channel is empty and still has senders, this method will wait until a message is
-    /// sent into the channel or until all senders get dropped.
+    /// If the channel is emtpy and still has senders, this method
+    /// will wait until a message is sent into it. Once all senders
+    /// have been dropped it will return `None`.
     ///
     /// # Examples
     ///
@@ -362,10 +363,13 @@ impl<T> Receiver<T> {
     /// task::spawn(async move {
     ///     s.send(1).await;
     ///     s.send(2).await;
+    ///     // Then we drop the sender
     /// });
     ///
     /// assert_eq!(r.recv().await, Some(1));
     /// assert_eq!(r.recv().await, Some(2));
+    ///
+    /// // recv() returns `None`
     /// assert_eq!(r.recv().await, None);
     /// #
     /// # })