refine the pithy print mechenism.

@@ -55,11 +55,14 @@ public:
 };
 
 /**
-* the stage is used for a collection of object to do print,
-* the print time in a stage is constant and not changed.
-* for example, stage #1 for all play clients, print time is 3s,
-* if there is 10clients, then all clients should print in 10*3s.
-* Usage:
+ * the stage is used for a collection of object to do print,
+ * the print time in a stage is constant and not changed,
+ * that is, we always got one message to print every specified time.
+ *
+ * for example, stage #1 for all play clients, print time is 3s,
+ * if there is 1client, it will print every 3s.
+ * if there is 10clients, random select one to print every 3s.
+ * Usage:
          SrsPithyPrint* pprint = SrsPithyPrint::create_rtmp_play();
          SrsAutoFree(SrsPithyPrint, pprint);
          while (true) {
@@ -70,7 +73,7 @@ public:
              }
              // read and write RTMP messages.
          }
-*/
+ */
 class SrsPithyPrint
 {
 private: