[css-conditional] Mention "CSS namespace" rather than "CSS interface" (#2603)

Author: heycam

css-conditional-3/Overview.bs

@@ -996,8 +996,8 @@ interface CSSDocumentRule : CSSConditionRule {
 -->
 
 
-<h3 id='the-css-interface'>
-The <code>CSS</code> interface, and the <code title=''>supports()</code> function</h3>
+<h3 id='the-css-namespace'>
+<a id='the-css-interface'>The <code>CSS</code> namespace, and the <code title=''>supports()</code> function</a></h3>
 
 The {{CSS}} namespace holds useful CSS-related functions that do not belong elsewhere.