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

**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