Document what is arg for in custom iterators

**Your Godot version:**
3.2.3

**Issue description:**
It would be useful for beginners to know what the `arg` argument is for in `custom iterator` methods, and how it can be used.
An example of using it would be helpful as well

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/getting_started/scripting/gdscript/gdscript_advanced.html#custom-iterators