doc/builtins/Motor: return from run_until_stalled

The return argument was not documented.

pybricks/builtins.py

@@ -263,14 +263,17 @@ def run_target(self, speed, target_angle, then=Stop.HOLD, wait=True):
 
     def run_until_stalled(self, speed, then=Stop.COAST, duty_limit=None):
         """Runs the motor at a constant speed until it
-        :ref:`stalls <stalled>`
+        :ref:`stalls <stalled>`.
 
         Arguments:
             speed (:ref:`speed`): Speed of the motor.
             then (Stop): What to do after coming to a standstill.
             duty_limit (:ref:`percentage`): Torque limit during this
                 command. This is useful to avoid applying the full motor
                 torque to a geared or lever mechanism.
+
+        Returns:
+            :ref:`angle`: Angle at which the motor becomes stalled.
         """
         pass