Shorten _register_synthetic_freq docstring under 120 chars

CI's ruff caught E501 on the one-line docstring in the test helper.

Author: hujc7

source/isaaclab_newton/test/cloner/test_rename_builder_labels.py

@@ -161,7 +161,7 @@ def setUp(self):
         self.mapping = torch.ones(1, len(self.worlds), dtype=torch.bool)
 
     def _register_synthetic_freq(self, builder, freq_name, world_attr_name, str_attr_names):
-        """Register a ``syn:<freq_name>`` custom frequency, a ``references="world"`` int companion, and one or more ``dtype=str`` columns at it."""
+        """Register a ``syn:<freq_name>`` frequency with one world int column and N string columns."""
         freq = f"syn:{freq_name}"
         builder.add_custom_frequency(newton.ModelBuilder.CustomFrequency(name=freq_name, namespace="syn"))
         builder.add_custom_attribute(