Make clear that free() doesn't set any references to it to null (in release builds)

[Demindiro]

Your Godot version:
3.2.2

Issue description:
While investigating a segmentation fault in a release build I realized one of the scripts was calling a method on an object that has been freed in the previous frame with queue_free. I assumed that all references to freed objects were automatically set to null since that is what seems to be happening in the editor (and debug builds).

That said, I'm not sure how is_instance_valid is supposed to work then (and it seems it indeed doesn't work at all). Is there something I'm misunderstanding?

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/classes/class_object.html#class-object-method-free

[Calinou]

See godotengine/godot#35534.

[Mickeon]

I don't recall this still being an issue, anymore. Should this be closed or kept for legacy reasons?

[Calinou]

Closing in favor of godotengine/godot-proposals#1589. See the last comment in that proposal.