Update channels.md (#254)

* Update channels.md

Adding speaker notes explaining why `send` and `recv` can fail.

* Explicitly mention that the channel is closed when the sender/receiver is dropped

Co-authored-by: Martin Geisler <martin@geisler.net>

src/concurrency/channels.md

@@ -21,3 +21,9 @@ fn main() {
     println!("Received: {:?}", rx.recv());
 }
 ```
+
+<details>
+
+* `send()` and `recv()` return `Result`. If they return `Err`, it means the counterpart `Sender` or `Receiver` is dropped and the channel is closed.
+
+</details>