[css-cascade-4] Mention aliasing mechanism in Additions since L3 section.

Author: fantasai

css-cascade-4/Overview.bs

@@ -1130,6 +1130,7 @@ Additions Since Level 3</h3>
 
 	* Introduced ''revert'' keyword, for rolling back the cascade.
 	* Introduced ''supports()'' syntax for supports-conditional ''@import'' rules.
+	* Defined the property two aliasing mechanisms CSS uses to support legacy syntaxes. See See [[#aliasing]].
 	* Added definition of how scoped styles would cascade
 		(deferred from Level 3)