Update proposals/hasCrossSiteAncestor.md

Update details on when an error would be returned.

Co-authored-by: Rob Wu <[email protected]>

proposals/hasCrossSiteAncestor.md

@@ -118,7 +118,7 @@ A Promise that will be fulfilled with a `Cookie.partitionKey` object that matche
 ##### Error conditions
 - If host permissions are not granted for the document whose partitionkey is getting queried an error will be returned.
 - When the parameters passed do not correspond to an existing frame, an error will be returned.
-- If the partitionKey that would be associated with the frame can not be serialized an error will be returned. This can happen when the origin associated with the topLevelSite is opaque or if the underlying key associated with the frame has a nonce.
+- If the partitionKey that would be associated with the frame can not be serialized an error will be returned. This can happen when the origin associated with the topLevelSite is opaque or if the underlying key associated with the frame has a nonce. This is comparable to situations where `document.cookie` would throw.
 
 ### New Permissions
 No new permissions are required.