[css-view-transitions-1] Clarify that root snapshot captures more than just the element. #9138 (#9152)

Co-authored-by: Khushal Sagar <63884798+khushalsagar@users.noreply.github.com>

Author: fantasai

css-view-transitions-1/Overview.bs

@@ -1635,7 +1635,9 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 
 		1. If |element| is the [=document element=], then:
 
-			1. Render the region of the |element| and the [=/document=]'s [=Document/top layer=] that intersects the [=snapshot containing block=],
+			1. Render the region of document
+				(including its [=canvas background=] and any [=Document/top layer=] content)
+				that intersects the [=snapshot containing block=],
 				on a transparent canvas the size of the [=snapshot containing block=],
 				following the [=capture rendering characteristics=], and these additional characteristics:
 
@@ -1663,7 +1665,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 						</figcaption>
 					</figure>
 
-			1. Return the canvas as an image.
+			1. Return this canvas as an image.
 				The natural size of the image is equal to the [=snapshot containing block=].
 
 		1. Otherwise:
@@ -1673,7 +1675,7 @@ urlPrefix: https://wicg.github.io/navigation-api/; type: interface;
 				over an infinite transparent canvas,
 				following the [=capture rendering characteristics=].
 
-			1. Return the portion of the canvas that includes |element|'s [=ink overflow rectangle=] as an image.
+			1. Return the portion of this canvas that includes |element|'s [=ink overflow rectangle=] as an image.
 				The [=natural dimensions=] of this image must be those of its [=principal box|principal=] [=border box=],
 				and its origin must correspond to that [=border box=]'s origin,
 				such that the image represents the contents of this [=border box=]