Clarify that resources must be made unique

Most properties of instances can be edited separately, but resources are shared by default. Unless the PhysicsMaterial of a RigidBody2D is made unique on an instance, editing its properties will affect other instances.

https://www.reddit.com/r/godot/comments/c3k1et/stuck_on_bouncing_ball_tutorial/

getting_started/step_by_step/instancing.rst

@@ -105,7 +105,9 @@ much more bouncy. Because the instanced balls are based on the saved scene,
 changes to that scene will affect all instances.
 
 You can also adjust individual instances. Set the bounce value back to ``0``
-and then in the ``Main`` scene, select one of the instanced balls. Set its
+and then in the ``Main`` scene, select one of the instanced balls. Resources
+like ``PhysicsMaterial`` are shared between instances by default, so we need
+to make it unique. Click on the down arrow and select "Make Unique". Set its
 ``Bounce`` to ``1`` and press "Play".
 
 .. image:: img/instancing_property.png