Clarify the Bevy UI and Taffy boundary

## How can Bevy's documentation be improved?

There are several Bevy UI features that users naturally assume are handled by Taffy, such as:
* Responsive coords
* Border radius
* Z-Index
* Overflow clipping
* Outlines

But all of these are resolved internally in Bevy, either before or after the Taffy layout pass in `ui_layout_system`.

To make Bevy UI easier to understand for both users and contributors, the module's documentation should clearly explain which responsibilities belong to Taffy and which are handled by Bevy itself.