add comment for stopping the simulation

ArmNavigation/n_joint_arm_to_point_control/NLinkArm.py

@@ -51,6 +51,7 @@ class NLinkArm(object):
 
     def plot(self):  # pragma: no cover
         plt.cla()
+        # for stopping simulation with the esc key.
         plt.gcf().canvas.mpl_connect('key_release_event',
                 lambda event: [exit(0) if event.key == 'escape' else None])