Add a note on updating command timestamp (moveit#500)

doc/arm_jogging/arm_jogging_tutorial.rst

@@ -61,7 +61,7 @@ Launch the jog node. This example uses commands from a `SpaceNavigator <https://
 
 If you do not have a SpaceNav 3D mouse, you can publish example jogging commands from the command line with: ::
 
-    rostopic pub -r 100 /jog_server/delta_jog_cmds geometry_msgs/TwistStamped "header: auto
+    rostopic pub -r 100 -s /jog_server/delta_jog_cmds geometry_msgs/TwistStamped "header: auto
     twist:
       linear:
         x: 0.0
@@ -72,6 +72,8 @@ If you do not have a SpaceNav 3D mouse, you can publish example jogging commands
         y: 0.0
         z: 0.0"
 
+The `-r 100` sends new commands at a 100 Hz rate. The combination of `-s` and `auto` updates the timestamp automatically.
+
 Settings
 --------
 User-configurable settings of the jog node are well-documented in ``jog_arm/config/ur_simulated_config.yaml``.