Add a comment explaining the xacro parameters.

Author: jskinn

rtb-data/rtbdata/xacro/puma560_description/urdf/puma560_robot.urdf.xacro

@@ -10,6 +10,12 @@
 
   <xacro:property name="M_PI" value="3.14159265"/>
 
+  <!--
+  List of distance parameters encoding the model (in meters).
+  Each name has two indices, first to the link number, and then to the joint.
+  So the distances between link2 and joint1  is `l2_1`, and link4 to joint 3 is `l4_3`.
+  Link 4 has two purpendicular offsets to joint 4, denoted l4_4y and l4_4z.
+  -->
   <xacro:property name="l1_1" value="0.54864"/>
   <xacro:property name="l2_1" value="0.07493"/>
   <xacro:property name="l2_2" value="0.14224"/>