Add docstring for Widget.DEBUG_LAYOUT_ENABLED #3948
Description
What is the problem or limitation you are having?
The property DEBUG_LAYOUT_ENABLED at https://toga.beeware.org/en/stable/reference/api/widgets/widget/#toga.Widget.DEBUG_LAYOUT_ENABLED has no docstring. It may be a bit confusing.
Describe the solution you'd like
A docstring backlinking it to the topic guide on debugging. That way we get more advertisement for the feature and also less confusion.
Describe alternatives you've considered
Not really; no docstirng looks weird.
Additional context
No response