Point to docstring for case-by-case

Author: reivilibre

synapse/storage/databases/main/thread_subscriptions.py

@@ -253,6 +253,7 @@ def _subscribe_user_to_thread_txn(
                 # is good enough (either we already have a manual subscription,
                 # or we requested an automatic subscription)
                 # In that case, nothing to change here.
+                # (See docstring for case-by-case explanation)
                 return None
 
             if not subscribed and requested_automatic: