Clarify Xacro and package URI handling

Author: sylvesterkaczmarek

docs/source/api/lab/isaaclab.sim.converters.rst

@@ -54,9 +54,11 @@ URDF Converter
        xacro path/to/robot.urdf.xacro > path/to/robot.urdf
        uv run python scripts/tools/convert_urdf.py path/to/robot.urdf path/to/output_dir
 
-    Any Xacro arguments and ROS package substitutions must therefore be resolved during the Xacro expansion
-    step. The resulting URDF can then use the normal :class:`UrdfConverterCfg` options for collision geometry,
-    joint drives, fixed-joint merging, and USD output.
+    Xacro expansion resolves macros and Xacro arguments, but it does not generally resolve ``package://`` mesh
+    URLs in the generated URDF. Rewrite those URLs to resolvable filesystem paths before using the CLI, or use
+    the Python API and provide package mappings through :attr:`UrdfConverterCfg.ros_package_paths`. The resulting
+    URDF can then use the normal :class:`UrdfConverterCfg` options for collision geometry, joint drives,
+    fixed-joint merging, and USD output.
 
 .. autoclass:: UrdfConverter
     :members: