[css-text-3] Strengthen language about conformant use of text-transform

As per w3c#8279

Author: frivoal

css-text-3/Overview.bs

@@ -584,9 +584,9 @@ Case Transforms: the 'text-transform' property</h3>
 	text-transform/text-transform-upperlower-105.html
 	</wpt>
 
-	Note: The 'text-transform' property only affects the presentation layer;
-	correct casing for semantic purposes is expected to be represented
-	in the source document.
+	Advisement: Authors must not rely on 'text-transform' for semantic purposes;
+	rather the correct casing and semantics should be encoded
+	in the source document text and markup.
 
 	<wpt>
 	text-transform/text-transform-copy-paste-001-manual.html