[popover] Component z-index setting

# Docs feedback
Popover z-index setting

## How can we improve Base UI documentation?
Currently, it's hard to understand how to change the z-index setting on the `Popover` component to make it appear above other elements that have a z-index value set. There's nothing about this in the documentation, and you have to dig into the source code to get insight into how everything works. I wouldn't have assumed that the `Popover.Positioner` component's classname could have a tailwind CSS utility like z-50 or any other z-value to make it appear over other elements. If you could add something to the documentation explaining how to do this, it would save time for anyone looking for this solution.