[css-highlight-api] Add comment about the default numerical priority for custom highlights.

Author: sanketj

css-highlight-api-1/Overview.bs

@@ -352,8 +352,10 @@ Priority of Overlapping Highlights</h4>
 	This is used to determine the stacking order of the corresponding [=highlight overlay=]
 	during painting operations (see [[#painting]]).
 	A higher [=priority=] results in being above in the stacking order.
+	A custom highlight will have a default numerical priority of 0
+	if its {{Highlight/priority}} attribute has not been explicitly set.
 
-	When two ore more [=custom highlights=] have the same numerical priority,
+	When two or more [=custom highlights=] have the same numerical priority,
 	the one most recently [=registered=] has the higher effective [=priority=].
 
 	<div class=example id=overlap-highlight-ex>