GM: allow enabling down to a stop for Silverado/Sierra (#28708)

* Silverado: allow enable down to 0 without user brake

* update docs

* firmly

* better comments, fix docs

* revert docs

selfdrive/car/gm/interface.py

@@ -224,6 +224,11 @@ class CarInterface(CarInterfaceBase):
       ret.steerRatio = 16.3
       ret.centerToFront = ret.wheelbase * 0.5
       tire_stiffness_factor = 1.0
+      # On the Bolt, the ECM and camera independently check that you are either above 5 kph or at a stop
+      # with foot on brake to allow engagement, but this platform only has that check in the camera.
+      # TODO: check if this is split by EV/ICE with more platforms in the future
+      if ret.openpilotLongitudinalControl:
+        ret.minEnableSpeed = -1.
       CarInterfaceBase.configure_torque_tune(candidate, ret.lateralTuning)
 
     elif candidate == CAR.EQUINOX: