[css-fonts-4] Clarify ignored font-family descriptor, w3c#6916

Author: svgeesus

css-fonts-4/Overview.bs

@@ -7053,7 +7053,9 @@ Font family: the 'font-family!!descriptor' descriptor</h4>
     The value of this descriptor  means that only named font families are allowed
     and rules that include generic fonts in the list of font families
     are syntax errors.
-    If syntax errors occur within the font family list, the descriptor must be ignored.
+    If syntax errors occur within the font family list, 
+	the descriptor must be ignored
+	(will still be in the CSS OM, but will not match any font families).
 
 <h4 id="base-palette-desc">
 Specifying the base palette: the 'base-palette' descriptor</h4>