[css-mixins-1] Disambiguate dfn "invalid at computed-value time" (#11443)

It's defined in both css-values-5 and css-variables-2,
according to bikeshed.

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>
Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: 3 people

css-mixins-1/Overview.bs

@@ -21,6 +21,7 @@ spec:css-syntax-3; type:dfn; text:declaration
 spec:css-syntax-3; type:dfn; text:descriptor;
 spec:css-values-4; type:dfn; text:keyword;
 spec:css-values-4; type:dfn; text:identifier;
+spec:css-values-5; type:dfn; text:invalid at computed-value time
 spec:cssom-1; type:dfn; text:specified order;
 </pre>