[css-text-3] Clarify that Default_ignorable is ignored for text rendering, not for all of CSS. whatwg/html#4854

Author: fantasai

css-text-3/Overview.bs

@@ -1008,7 +1008,8 @@ Text Processing</h3>
     generate a visual representation of its code point value,
     or use some other method to provide an appropriate visible glyph.
     As required by [[!UNICODE]],
-    unsupported <code>Default_ignorable</code> characters must be ignored for rendering.
+    unsupported <code>Default_ignorable</code> characters
+    must be ignored for text rendering.
 
     <wpt>
       white-space/control-chars-000.html