Improve Folly thread pool executor documentation for running task

Summary:
The expire Callback is not intuitive to understand that it means if the task waittime has expired or the task is running for more than the expiration time

So adding a comment

Reviewed By: Gownta

Differential Revision: D49432292

fbshipit-source-id: e95cd1b589745f0e1145c7b0184c4bc0f49c3a43

folly/executors/ThreadPoolExecutor.h

@@ -74,6 +74,14 @@ class ThreadPoolExecutor : public DefaultKeepAliveExecutor {
   ~ThreadPoolExecutor() override;
 
   void add(Func func) override = 0;
+  /**
+   * If func doesn't get started within expiration time after its enqueued,
+   * expireCallback will be run
+   *
+   * @param func  Main function to be executed
+   * @param expiration Maximum time to wait for func to start execution
+   * @param expireCallback If expiration limit is reached, execute this callback
+   */
   virtual void add(
       Func func, std::chrono::milliseconds expiration, Func expireCallback);