[css-shapes] clarify default <position> serialization

Author: astearns

css-shapes/Overview.bs

@@ -483,7 +483,9 @@ Serialization of Basic Shapes</h3>
 	joining space-separated tokens with a single space,
 	and following each serialized comma with a single space.
 
-	The <<position>> values in ''ellipse()'' and ''circle()''
+	The <<position>> values 
+	(including the defaults) 
+	in ''ellipse()'' and ''circle()''
 	serialize to their 2- and 4-value forms only,
 	preferring the 2-value form
 	when it can be expressed without calc(),