[view-transitions-2] Do not access box fragments on things that don't fragment

Neither elements nor captured elements can fragment. Only boxes do that.
I'm making this care about the principal box here, since fragmenting any
other boxes that might be created for an element inside of its principal
box will also fragment the principal box.

Author: Psychpsyo

css-view-transitions-2/Overview.bs

@@ -2536,7 +2536,7 @@ It has the following [=struct/items=]:
 				has ''contain: view-transition'',
 				[=continue=].
 
-			1. If |element| has more than one [=box fragment=], then [=continue=].
+			1. If |element|'s [=principal box=] has more than one [=box fragment=], then [=continue=].
 
 				Note: We might want to enable transitions for fragmented elements in future versions.
 				See [#8900](https://github.com/w3c/csswg-drafts/issues/8900).
@@ -2626,7 +2626,7 @@ It has the following [=struct/items=]:
 				or |element| is [=element-not-rendered|not rendered=],
 				then [=continue=].
 
-			1. If |element| has more than one [=box fragment=], then [=continue=].
+			1. If |element|'s [=principal box=] has more than one [=box fragment=], then [=continue=].
 
 			1. If |usedTransitionNames| [=list/contains=] |transitionName|,
 				then return failure.
@@ -3398,7 +3398,7 @@ To <dfn>adjust nested view transition group transform</dfn> given a |transform|
 
 					- |capturedElement|'s [=new element=] is [=element-not-rendered|not rendered=].
 
-					- |capturedElement| has more than one [=box fragment=].
+					- |capturedElement|'s [=new element=]'s [=principal box=] has more than one [=box fragment=].
 
 					Note: Other rendering constraints are enforced via |capturedElement|'s [=new element=] being [=captured in a view transition=].