Failsafe exit setting for awful.popup in doc example

[elig0n]

The first awful.widget.layoutlist popup example does not show how to bind the popup to a widget or a button press.

Nevertheless the popup is automatically shown upon creation so I would consider adding visible = false to avoid that and let a binding show it.

Also it is not removed from display unless the user picks something, not even after an awesome restart, so I think adding this option which lets the unaware user click the right mouse button to make the popup disappear without being forced to select a layout should be set as a sane default/protection.

I would also consider adding ontop so it is not covered by clients/more visible when active since it is usually an immediate type of action.