am7xxx-play: add a comment to explain why a run-time warning can be ignored

Using the AV_PIX_FMT_YUVJ420P pixel format results n a run-time warning
from swscaler:

[swscaler @ 0x5600046b3cc0] deprecated pixel format used, make sure you did set range correctly

However this cannot be changed because the mjpeg codec relies on it,
otherwise it fails with the following error message:

[mjpeg @ 0x560eea5f5360] Specified pixel format yuv420p is invalid or not supported

So just add a comment, to avoid loosing time figuring things out again
in the future.