Add more docs

ur_controllers/doc/index.rst

@@ -139,11 +139,13 @@ Advertised services
 .. _passthrough_trajectory_controller:
 
 ur_controllers/PassthroughTrajectoryController
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 This controller uses a ``control_msgs/FollowJointTrajectory`` action but instead of interpolating
 the trajectory on the ROS pc it forwards the complete trajectory to the robot controller for
-interpolation and execution. This way, the realtime requirements for the control PC.
+interpolation and execution. This way, the realtime requirements for the control PC can be
+massively decreased, since the robot always "knows" what to do next. That means that you should be
+able to run a stable driver connection also without a real-time patched kernel.
 
 Interpolation depends on the robot controller's implementation, but in conjunction with the
 ur_robot_driver it defaults to mimicking ros2_control's spline interpolation. So, any trajectory
@@ -165,6 +167,11 @@ controller basically tracks the scaled time instead of the real time.
    the expected time depending on the simulation host's resources. This effect will not be present
    when using a real UR arm.
 
+.. note::
+
+   This controller can currently only be used with URSim or a real UR robot. Neither mock hardware
+   nor gazebo support this type of trajectory interfaces at the time being.
+
 Tolerances
 """"""""""