Document that CollisionShape(2D) instances share a resource, and that 'Local to Scene' can override this

**Your Godot version:**
3.2.4 RC 2

**Issue description:**
It isn't obvious that instanced `CollisionShape` nodes will share a resource, or why.
The `Make Sub-Resources Unique` checkbox seems to have no effect on this issue.
Something like this should probably be added at the top of the `CollisionShape` and `CollisionShape2D` docs:

_For performance reasons, all instanced `CollisionShape(2D)` nodes share the same resource.
To override this and create unique resources for each instance, check the `Local to Scene` box in the inspector._

**Issues:**
https://github.com/godotengine/godot/issues/38066
https://www.reddit.com/r/godot/comments/ljd49i/instanced_objects_interact_as_one_object/
https://godotengine.org/qa/95271/give-unique-size-to-different-collisionshape2d-instances
https://github.com/godotengine/godot-proposals/issues/317

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_collisionshape.html
https://docs.godotengine.org/en/stable/classes/class_collisionshape2d.html