Add a missing comment documenting the changes done in PR 2841.

common/lib/xmodule/xmodule/js/src/video/03_video_player.js

@@ -558,6 +558,17 @@ function (HTML5Video, Resizer) {
             }
         );
 
+        // Because of a recent change in the YouTube API (not documented), sometimes
+        // HTML5 mode loads after Flash mode has been loaded. In this case we have
+        // multiple speeds available but the variable `this.currentPlayerMode` is
+        // set to "flash". This is impossible because in Flash mode we can have
+        // only one speed available. Therefore we must execute the following code
+        // block if we have multiple speeds or if `this.currentPlayerMode` is set to
+        // "html5". If any of the two conditions are true, we then set the variable
+        // `this.currentPlayerMode` to "html5".
+        //
+        // For more information, please see the PR that introduced this change:
+        //     https://github.com/edx/edx-platform/pull/2841
         if (
             (this.currentPlayerMode === 'html5' || availablePlaybackRates.length > 1) &&
             this.videoType === 'youtube'