[css-fonts] [palettes] font-palette cannot be expanded in the future

Fixes #6633.

Author: litherum

css-fonts-4/Overview.bs

@@ -6293,6 +6293,8 @@ Animation type: discrete
 		If no applicable ''@font-palette-values'' rule is present,
 		this value behaves as ''font-palette/normal''.
 
+        <<palette-identifier>> is parsed as a <<dashed-ident>>.
+
         Note: Such a CSS-defined palette may
         be a reference to an existing palette already present in the font,
         be a modification of an existing palette already present in the font,