Add inline documentation to move_along_path (#309)

src/picknik_ur_mock_hw_config/objectives/move_along_path.xml

@@ -1,8 +1,9 @@
 <?xml version='1.0' encoding='UTF-8'?>
 <root BTCPP_format="4" main_tree_to_execute="Move Along Path">
   <!--//////////-->
-  <BehaviorTree ID="Move Along Path" _description="Moves the robot tip on a rectangular path." _favorite="true" _hardcoded="false">
+  <BehaviorTree ID="Move Along Path" _description="Example of using Cartesian control to draw a rectangle in the air." _favorite="true" _hardcoded="false">
     <Control ID="Sequence" name="TopLevelSequence">
+      <!--First manually create an example Cartesian reference trajectory to then feed into the planner-->
       <Control ID="Sequence">
         <Action ID="CreateStampedPose" reference_frame="base_link" position_xyz="-0.4;0.4;-0.0" orientation_xyzw="0.0;1.0;0.0;0.0" stamped_pose="{stamped_pose}"/>
         <Action ID="AddPoseStampedToVector" input="{stamped_pose}" vector="{pose_stamped_vector}"/>
@@ -17,10 +18,13 @@
       </Control>
       <Decorator ID="KeepRunningUntilFailure">
         <Control ID="Sequence">
+          <!--Move to starting position, before planning and executing Cartesian path-->
           <SubTree ID="Move to Waypoint" waypoint_name="Look at Right Table" joint_group_name="manipulator" controller_names="/joint_trajectory_controller" planner_interface="moveit_default"/>
+          <!--Plan and validate the trajectory-->
           <Action ID="PlanCartesianPath" path="{pose_stamped_vector}" planning_group_name="manipulator" tip_link="grasp_link" tip_offset="0.0;0.0;0.0" position_only="false" blending_radius="0.02" velocity_scale_factor="1.0" acceleration_scale_factor="1.0" trajectory_sampling_rate="100" joint_trajectory_msg="{joint_trajectory_msg}"/>
           <Action ID="GetCurrentPlanningScene" planning_scene_msg="{planning_scene_msg}"/>
           <Action ID="ValidateTrajectory" planning_scene_msg="{planning_scene_msg}" planning_group_name="manipulator" joint_trajectory_msg="{joint_trajectory_msg}" joint_space_step="0.05" cartesian_space_step="0.02"/>
+          <!--Finally, execute the trajectory on the actual robot-->
           <Action ID="ExecuteFollowJointTrajectory" execute_follow_joint_trajectory_action_name="/joint_trajectory_controller/follow_joint_trajectory" joint_trajectory_msg="{joint_trajectory_msg}"/>
         </Control>
       </Decorator>