sync: add panic docs for tokio::sync::broadcast::channel (#4622)

tokio/src/sync/broadcast.rs

@@ -425,6 +425,11 @@ const MAX_RECEIVERS: usize = usize::MAX >> 2;
 ///     tx.send(20).unwrap();
 /// }
 /// ```
+///
+/// # Panics
+///
+/// This will panic if `capacity` is equal to `0` or larger
+/// than `usize::MAX / 2`.
 pub fn channel<T: Clone>(mut capacity: usize) -> (Sender<T>, Receiver<T>) {
     assert!(capacity > 0, "capacity is empty");
     assert!(capacity <= usize::MAX >> 1, "requested capacity too large");