add a comment (#4789)

Co-authored-by: Richard Kuo (Onyx) <rkuo@onyx.app>

backend/onyx/connectors/slack/connector.py

@@ -381,6 +381,9 @@ def _message_to_doc(
     filtered_thread: ThreadType | None = None
     thread_ts = message.get("thread_ts")
     if thread_ts:
+        # NOTE: if thread_ts is present, there's a thread we need to process
+        # ... otherwise, we can skip it
+
         # skip threads we've already seen, since we've already processed all
         # messages in that thread
         if thread_ts in seen_thread_ts: