Add a comment explaining the xacro parameters.
<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"/>
