message: Clarify lifetime requirements on Envoy::Event::Timer
diff: @@ -16,7 +16,8 @@ namespace Event {
  using TimerCb = std::function<void()>;
  /**
  - * An abstract timer event. Free the timer to unregister any pending timeouts.
  + * An abstract timer event. Free the timer to unregister any pending timeouts. Must be freed before
  + * the dispatcher is torn down.
  */
  class Timer {
  public:
  