control: complement comment for subscribe API
control: complement comment for subscribe API

When minus value is given as a parameter, the API returns whether
the caller has already subscribed or not.

This commit adds a comment about this so as better documentation.

Signed-off-by: Takashi Sakamoto <d5ec7d0cd8074757a1f5eb708bc105c3065a65ea@sakamocchi.jp>
Signed-off-by: Takashi Iwai <4596b3305151c7ee743192a95d394341e3d3b644@suse.de>

lgirdwood/alsa-lib,tiwai/alsa-lib,mengdonglin/alsa-lib,lgirdwood/alsa-lib,tiwai/alsa-lib,Distrotech/alsa-lib,takaswie/alsa-lib,takaswie/alsa-lib,Distrotech/alsa-lib,alsa-project/alsa-lib,lgirdwood/alsa-lib,mengdonglin/alsa-lib,lgirdwood/alsa-lib,takaswie/alsa-lib,takaswie/alsa-lib,alsa-project/alsa-lib,mengdonglin/alsa-lib,mengdonglin/alsa-lib,tiwai/alsa-lib,zedongchen/alsa-lib,alsa-project/alsa-lib,lgirdwood/alsa-lib,zedongchen/alsa-lib,Distrotech/alsa-lib,Distrotech/alsa-lib,zedongchen/alsa-lib,alsa-project/alsa-lib,Distrotech/alsa-lib,zedongchen/alsa-lib,takaswie/alsa-lib,mengdonglin/alsa-lib,zedongchen/alsa-lib,tiwai/alsa-lib