[css-text-decor-3] Define position of emphasis marks wrt empty and auto-hidden annotations per Shinyu Murakami's advice, <https://lists.w3.org/Archives/Public/www-style/2017Apr/0020.html>.

Author: fantasai

css-text-decor-3/Overview.bs

@@ -789,6 +789,8 @@ Emphasis Mark Position: the 'text-emphasis-position' property</h3>
   <p>If emphasis marks are applied to characters
     for which ruby is drawn in the same position as the emphasis mark,
     the emphasis marks are placed outside the ruby.
+    This includes <a href="https://www.w3.org/TR/css-ruby-1/#autohide">auto-hidden</a>
+    and empty <a>ruby annotations</a>.
 
   <div class="figure">
     <p><img