zh doc: Sync Audio with en doc.

docs/api/zh/audio/Audio.html

@@ -90,6 +90,9 @@
 		<h3>[property:Number offset]</h3>
 		<p>音频开始播放的偏移时间. 和[link:https://developer.mozilla.org/en-US/docs/Web/API/AudioBufferSourceNode/start AudioBufferSourceNode.start]()的*offset*参数一样. 默认为 *0*.</p>
 
+		<h3>[property:Number duration]</h3>
+		<p>覆盖音频的持续时间。与[link:https://developer.mozilla.org/en-US/docs/Web/API/AudioBufferSourceNode/start AudioBufferSourceNode.start]()中的*duration*属性相同。默认为*undefined*，以用于播放整个buffer。</p>
+
 		<h3>[property:String source]</h3>
 		<p>使用 [link:https://developer.mozilla.org/en-US/docs/Web/API/AudioContext/createBufferSource AudioContext.createBufferSource]()创建的[link:https://developer.mozilla.org/en-US/docs/Web/API/AudioBufferSourceNode AudioBufferSourceNode].</p>