[css-fonts] [palettes] base-palette and override-color should have their integer values be required to be nonnegative

Fixes w3c#6626.

Author: litherum

css-fonts-4/Overview.bs

@@ -6466,7 +6466,7 @@ Specifying the base palette: the 'base-palette' descriptor</h4>
 
     <pre class='descdef'>
     Name: base-palette
-    Value: <<integer>> | <<string>>
+    Value: <<integer [0, ∞]>> | <<string>>
     For: @font-palette-values
     Initial: N/A
     </pre>
@@ -6528,7 +6528,7 @@ Overriding a color from a palette: The 'override-color!!descriptor' descriptor</
 
     <pre class='descdef'>
     Name: override-color
-    Value: [ [ <<string>> | <<integer>> ] <<color>> ]#
+    Value: [ [ <<string>> | <<integer [0, ∞]>> ] <<color>> ]#
     For: @font-palette-values
     Initial: N/A
     </pre>