Mention the "Items" button that appears in the top bar when you click on an OptionButton

**Your Godot version:**
3.4-beta4
**Issue description:**
There's an "Items" button in the top bar that appears when you click on an OptionButton that you can use to add items to an OptionButton. This isn't mentioned anywhere on the OptionButton page.
**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/classes/class_optionbutton.html