[traffic] Documentation and comments

Signed-off-by: mvglasow <michael -at- vonglasow.com>

map/traffic_manager.cpp

@@ -136,6 +136,7 @@ void TrafficManager::SetEnabled(bool enabled)
 
 void TrafficManager::Clear()
 {
+  // TODO what should we do about subscriptions?
   {
     std::lock_guard<std::mutex> lock(m_mutex);
 

map/traffic_manager.hpp

@@ -600,6 +600,11 @@ private:
   std::vector<MwmSet::MwmId> m_requestedMwms;
 #endif
 
+  /**
+   * @brief Mutex for access to shared members.
+   *
+   * Threads which access shared members (see documentation) must lock this mutex while doing so.
+   */
   std::mutex m_mutex;
 
   /**