[css-fonts-4] Fix w3c#1736

Author: svgeesus

css-fonts-4/Overview.bs

@@ -197,11 +197,16 @@ corresponding issues of case sensitivity are described below in the
 <h4 id="generic-font-families">
 Generic font families</h4>
 
-All five generic font families are defined to exist in all CSS
-implementations (they need not necessarily map to five distinct actual
-fonts). User agents should provide reasonable default choices for the
-generic font families, which express the characteristics of each family
-as well as possible within the limits allowed by the underlying
+All nine generic font families must always result in at least
+one matched font face, for all CSS implementations. However, the
+generics may be composite faces (with different typefaces based
+on such things as the Unicode range of the character, the language of the containing
+element, user preferences and system settings, among others). They are also
+not guaranteed to always be different from each other.
+
+User agents should provide reasonable default choices for the
+generic font families, which express the characteristics of each
+family as well as possible, within the limits allowed by the underlying
 technology. User agents are encouraged to allow users to select
 alternative choices for the generic fonts.