[css-ui] Allow visually repositioning carets

Follow up on #10289

Author: frivoal

css-ui-4/Overview.bs

@@ -1360,6 +1360,9 @@ Shape of the insertion caret: 'caret-shape'</h4>
 	even if that means means painting
 	past the bounds of the [=scrollable overflow area=];
 	user agents may impose limits on this and clip as necessary.
+	User agents may also visually reposition a caret
+	in order to display it within the [=scrollport=]
+	if it would otherwise be invisible.
 
 	The stacking position of the [=caret=] is left undefined within the following constraints: