Added update method docs

src/sound/html5/HTML5AudioSound.js

@@ -291,6 +291,14 @@ var HTML5AudioSound = new Class({
         this.isPaused = false;
         this.pickAndPlayAudioTag();
     },
+    /**
+     * Update method called automatically by sound manager on every game step.
+     *
+     * @protected
+     * @method Phaser.Sound.HTML5AudioSound#update
+     * @param {number} time - The current timestamp as generated by the Request Animation Frame or SetTimeout.
+     * @param {number} delta - The delta time elapsed since the last frame.
+     */
     update: function (time, delta) {
         if (!this.isPlaying) {
             return;