Document caveat with @export_tool_button and RefCounted-inheriting classes in GDScript exports

[Calinou]

• See export_tool_button self-referencing resources godot-proposals#15276.

[dalexeev]

You can use code-example-good and code-example-bad, like here:

godot-docs/tutorials/scripting/gdscript/gdscript_styleguide.rst

Lines 124 to 143 in 98cd4a4

	**Good**:
	.. rst-class:: code-example-good
	::
	for i in range(10):
	print("hello")
	**Bad**:
	.. rst-class:: code-example-bad
	::
	for i in range(10):
	print("hello")
	for i in range(10):
	print("hello")

[dalexeev]

Suggested change

	@export_tool_button("My Button", "Callable") var mybutton = func():
	self.some_var = some_func()
	**Good:**
	::
	@tool
	extends Resource
	@export_tool_button("My Button", "Callable") var my_button:
	get: return func():
	self.some_var = some_func()
	@export_tool_button("My Button") var my_button = func ():
	some_var = some_func()
	**Good:**
	::
	@tool
	extends Resource
	@export_tool_button("My Button") var my_button:
	get: return func ():
	some_var = some_func()

[dalexeev]

Suggested change

	or :ref:`Callable.unbind() <class_Callable_method_bind>`.
	or :ref:`Callable.unbind() <class_Callable_method_bind>`.
	Alternatively, you can use a getter: