add a comment for list_channel_closures

Author: EthanYuan

Diff for: mutiny-wasm/src/lib.rs

@@ -970,7 +970,7 @@ impl MutinyWallet {
             .list_channel_closures()
             .await?
             .into_iter()
-            .filter(|closure| closure.timestamp != 0)
+            .filter(|closure| closure.timestamp != 0) // filter out placeholder closures
             .map(Into::into)
             .collect();
         channel_closures.sort();