MediaPlayer: add comments of restriction for setNextMediaPlayer.

     * (i.e. reaches the end of the stream).

     * The media framework will attempt to transition from this player to

     * the next as seamlessly as possible. The next player can be set at

     * any time before completion. The next player must be prepared by the

     * any time before completion, but shall be after setDataSource has been

     * called successfully. The next player must be prepared by the

     * app, and the application should not call start() on it.

     * The next MediaPlayer must be different from 'this'. An exception

     * will be thrown if next == this.