Document unexpected behavior when exported arrays are shared

[me2beats]

Your Godot version:
3.5 beta, 3.4 stable

Issue description:

class Test:
	extends Node
	export var x = []

func _ready():
	var test1 = Test.new()
	var test2 = Test.new()
	test1.x.append(0)
	print(test2.x)

godotengine/godot#48038

This is unexpected because I assume exported vars are kinda regular vars (properties), not constants.

URL to the documentation page (if already existing):
gdscript_exports is the right place to clarify this behavior imo
https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_exports.html

[zacryol]

The specific issue, as of the last time I checked, was:

If an export var Array/Dict has a default value set in code, then any instance that does not have that value changed to something else via the editor will share a reference to that array/dict.

Seems to be fixed in master though.