[css-text-3] Make sure control characters really are visible

Closes w3c#1990

Author: frivoal

css-text-3/Overview.bs

@@ -598,6 +598,7 @@ Characters and Letters</h3>
     Control characters (<a>Unicode category</a> <code>Cc</code>)
     other than tab (U+0009), line feed (U+000A), form feed (U+000C), and carriage return (U+000D)
     must be rendered as a visible glyph
+    which the UA must synthethize if the glyphs found in the font are not visible
     and otherwise treated as any other character
     of the Other Symbols (<code>So</code>) <a>general category</a> and Common <a lt="Unicode script">script</a>.
     The UA may use a glyph provided by a font specifically for the control character,