[css-fonts-4] Explicitly note that generic(fangsong) is a script-specific, may-match generic w3c#4425

Author: svgeesus

css-fonts-4/Overview.bs

@@ -639,6 +639,9 @@ Generic font families</h4>
 			compared to a Song style.
 			Fang Song is often used for official Chinese Government documents.
 
+			Note: ''generic(fangsong)'' might not map to any locally installed font,
+			but if it does, that font will be in Fang Song style.
+
 	    <dt id="ui-serif-def"><dfn>ui-serif</dfn>
 	    <dd>
 	        This font family is used for the serif variant of the system's