update docs against 8661

Signed-off-by: Nwusongqing <wusongqing@huawei.com>

en/application-dev/media/audio-playback.md

@@ -8,6 +8,8 @@ You can use audio playback APIs to convert audio data into audible analog signal
 
 ![en-us_image_audio_state_machine](figures/en-us_image_audio_state_machine.png)
 
+**Note**: If the status is **Idle**, setting the **src** attribute does not change the status. In addition, after the **src** attribute is set successfully, you must call **reset()** before setting it to another value.
+
 
 
 **Figure 2** Layer 0 diagram of audio playback