[css-view-transitions-1] Clarify that the whole canvas is captured not just root box #9138
Description
Render the region of the element and the document's top layer that intersects the snapshot containing block, on a transparent canvas the size of the snapshot containing block
Generally speaking, rendering an element renders its principal box and not stuff outside it. But in this case we want to capture the entire canvas, including backgrounds and overflow that paint outside the root box, and that's not very clear from this wording.