[cssom] Make MediaList.item return serialization.

Fixes #600.

Author: upsuper

cssom/Overview.bs

@@ -478,9 +478,10 @@ Setting the {{MediaList/mediaText}} attribute must run these steps:
  value to the <a>collection of media queries</a>.
 </ol>
 
-The <dfn method for=MediaList>item(<var>index</var>)</dfn> method must return the media query in the <a>collection of media
-queries</a> given by <var>index</var>, or null, if <var>index</var> is greater than or equal to the number of media queries in the
-<a>collection of media queries</a>.
+The <dfn method for=MediaList>item(<var>index</var>)</dfn> method must return a
+<a lt="serialize a media query">serialization</a> of the media query in the <a>collection of media queries</a>
+given by <var>index</var>, or null, if <var>index</var> is greater than or equal to the number of media queries
+in the <a>collection of media queries</a>.
 
 The <dfn attribute for=MediaList>length</dfn> attribute must return the number of media queries in the <a>collection of media
 queries</a>.