message: add clarifying comment to watch_map.cc
diff: @@ -150,6 +150,7 @@ std::set<std::string> WatchMap::findAdditions(const std::vector<std::string>& ne
  newly_added_to_subscription.insert(name);
  watch_interest_[name] = {watch};
  } else {
  + // Add this watch to the already-existing set at watch_interest_[name]
  entry->second.insert(watch);
  }
  }
  