Add note about shadow casting and light cull masks in 3D lights and shadows (#4925)

Co-authored-by: Hugo Locurcio <hugo.locurcio@hugo.pro>

tutorials/3d/lights_and_shadows.rst

@@ -37,6 +37,9 @@ Each one has a specific function:
 -  **Specular**: Affects the intensity of the specular blob in objects affected by this light. At zero, this light becomes a pure diffuse light.
 -  **Bake Mode**: Sets the bake mode for the light. For more information see :ref:`doc_baked_lightmaps`
 -  **Cull Mask**: Objects that are in the selected layers below will be affected by this light.
+   Note that objects disabled via this cull mask will still cast shadows.
+   If you don't want disabled objects to cast shadows, adjust the ``cast_shadow`` property on the
+   GeometryInstance to the desired value.
 
 Shadow mapping
 ^^^^^^^^^^^^^^