Add comment explaining cancellation flow

Author: simolus3

packages/powersync_core/lib/src/sync/streaming_sync.dart

@@ -89,8 +89,18 @@ class StreamingSyncImplementation implements StreamingSync {
       final future = abort.abort();
       _internalCrudTriggerController.close();
 
-      // This makes the merged stream emit a done event, allowing us to break
-      // out of the loop.
+      // If a sync iteration is active, the control flow to abort is:
+      //
+      //  1. We close the non-line sync event stream here.
+      //  2. This emits a done event.
+      //  3. `addBroadcastStream` will cancel all source subscriptions in
+      //      response to that, and then emit a done event too. If there is an
+      //      error while cancelling the stream, it's forwarded by emitting an
+      //      error before closing.
+      //  4. We break out of the sync loop (either due to an error or because
+      //     all resources have been closed correctly).
+      //  5. `streamingSync` completes the abort controller, which we await
+      //     here.
       await _nonLineSyncEvents.close();
 
       // Wait for the abort to complete, which also guarantees that no requests