Document ``items`` in OptionButton/PopupMenu/ItemList

[MathiasBaumgartinger]

Your Godot version: 3.4.stable

Issue description:

As described in this issue: https://github.com/godotengine/godot-proposals/issues/4483, the items property in OptionButton/PopupMenu/ItemList is not at all documented (and thus even more misleading that it already is). A small documentation on this would be great.

URL to the documentation page (if already existing):

1. https://docs.godotengine.org/en/stable/classes/class_optionbutton.html
2. https://docs.godotengine.org/en/stable/classes/class_popupmenu.html
3. https://docs.godotengine.org/en/stable/classes/class_itemlist.html

[Calinou]

There is no items property for those nodes in 3.x, but a way to list the items should be documented instead.

(You linked to the /stable documentation, which only intends to document 3.x, not 4.0 until it's released.)