Add comment explaining what's going on here.

module_build_service/scheduler/main.py

@@ -159,6 +159,13 @@ class MessageWorker(threading.Thread):
             log.info("Calling %s" % idx)
             further_work = handler(conf, session, msg) or []
             log.info("Done with %s" % idx)
+
+            # Handlers can *optionally* return a list of fake messages that
+            # should be re-inserted back into the main work queue. We can use
+            # this (for instance) when we submit a new component build but (for
+            # some reason) it has already been built, then it can fake its own
+            # completion back to the scheduler so that work resumes as if it
+            # was submitted for real and koji announced its completion.
             for event in further_work:
                 log.info("  Scheduling faked event %r" % event)
                 self.incoming_work_queue.put(event)