[css-values-4] Define textual data types as not interpolable>

Author: fantasai

css-values-4/Overview.bs

@@ -460,6 +460,9 @@ Textual Data Types</h2>
 	<<custom-ident>> should be used instead.
 	It is provided as a convenience for defining other syntactic constructs.
 
+	All textual data types <a>interpolate</a> as <a>discrete</a>
+	and are <a>not additive</a>.
+
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