add comment about DoomFrames from unregistered dooms

src/doompanning.cc

@@ -341,7 +341,13 @@ void do_networking(ControllerContext &ctx)
                 #endif
             }
             else
+            {
+                // FIXME: this case happens if too many dooms are spawned at
+                // once: we miss some of their DoomState DP_DS_Ready messages
+                // and thus never register them. The dooms do however see our
+                // DP_DC_RunDoom commands so they start running.
                 log_trace("Received DoomFrame from unregistered doom (pid=%d)", pid);
+            }
         }
 
         nng_msg_free(msg);