[css-fonts-4] Clarify out-of-range palette indexes are ignored, fix w3c#7032

Author: svgeesus

css-fonts-4/Overview.bs

@@ -6548,6 +6548,7 @@ Overriding a colors from a palette: The 'override-colors!!descriptor' descriptor
     For each key/value pair in the value of this descriptor,
     the color with that key in the initial palette (i.e. by using the 'base-palette' descriptor)
     is overwritten by the color specified in this descriptor's value.
+	A key that is outside the range of indices of the initial palette is ignored.
 
     Palette index entries
     in the ''@font-palette-values/override-colors'' descriptor