CheckButton - link to example "How to use this control"

**Your Godot version:**
3.2.2

**Issue description:**
CheckButton - how the hell do you use it? How do you know if it is checked? The event does not pass in any kind of state.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_checkbutton.html