[css-color-adjust-1] Specify what keywords aren't allowed to be <custom-ident> so we're following the spec-author conformance rules in Values 3. ^_^

Author: tabatkins

css-color-adjust-1/Overview.bs

@@ -126,6 +126,8 @@ Opting Into a Preferred Color Scheme: the 'color-scheme' property {#color-scheme
 			authoring tutorials and references
 			should omit it from their materials.
 
+			''only'', ''color-scheme/auto'', ''light'', and ''dark'' are not valid <<custom-ident>>s in this property.
+
 			Repeating a keyword, such as ''color-scheme: light light'',
 			is valid but has no additional effect
 			beyond what the first instance of the keyword provides.