[css-text-3] Editorial tweak

"encoding different from Unicode" is nonsensical, since Unicode isn't
an encoding. We're actually speaking of UAs that have a different text
encoding *model*, so update the text to say so.

Author: frivoal

css-text-3/Overview.bs

@@ -393,9 +393,9 @@ Text Processing</h3>
 	UAs that support Unicode must adhere to all normative requirements
 	of the Unicode Core Standard,
 	except where explicitly overridden by CSS.
-	UAs that use a different encoding are not explicitly supported by the CSS specifications;
-	they are, however, expected to fulfill the same text handling requirements
-	by assuming an appropriate mapping between that encoding and Unicode.
+	UAs implemented on the basis of a non-Unicode text encoding model are still
+	expected to fulfill the same text handling requirements
+	by assuming an appropriate mapping and analogous behavior.
 
 	<wpt>
 	text-encoding/shaping-join-001.html