[css-fonts-4] Add rule to determine the winner when multiple override-colors fight

Author: litherum

css-fonts-4/Overview.bs

@@ -6584,6 +6584,10 @@ Overriding a colors from a palette: The 'override-colors!!descriptor' descriptor
 
     Integer values are zero-indexed.
 
+    If the keys of multiple distinct key/value pairs identify the same color index (either by name or by integer),
+    the first key is used for the purposes of rendering. However, for serialization purposes, both
+    key/value pairs are present.
+
     Using CSS Variables is explicitly expected to be valid in the value of this descriptor.
     CSS Variables are resolved in the context of the element to which the 'font-palette' property applies.