Mention that source fragments are XML/HTML syntax.

Author: fantasai

selectors3/Overview.html

@@ -271,6 +271,9 @@ <h3><a name=terminology></a>1.2. Terminology</h3>
   <p>Additional terminology is defined in the
   <a href="http://www.w3.org/TR/CSS21/conform.html#defs">Definitions</a>
   section of <a href="#refsCSS21">[CSS21]</a>.
+  Examples of document source code and fragments are given in XML
+  <a name="refsXML10">[XML]</a> or HTML <a href="refsHTML40">[HTML40]</a>
+  syntax.
 
   <h3><a name=changesFromCSS2></a>1.3. Changes from CSS2</h3>