[css-text-3] Add a note referencing UAX9's rule about rendering tab characters.

Author: fantasai

css-text-3/Overview.bs

@@ -756,6 +756,9 @@ Characters and Letters</h3>
         <dfn lt="tab stop">Tab stops</dfn> occur at points that are multiples of the tab size
         from the block's starting content edge.
         The tab size is given by the 'tab-size' property.
+
+        Note: See [[UAX9]] for <a href="http://unicode.org/reports/tr9/#L1">rules on how U+0009 tabulation interacts with bidi</a>.
+
       <li>A sequence at the end of a line
         (ignoring any intervening <a>inline box</a> boundaries)
         of <a>collapsible</a> spaces (U+0020)