pcm: document interaction of drain silence and sw silence parameter

alsa-project/alsa-lib pull request #381 was opened from z-s-e:

I'm not sure this is the best way to describe the interaction, but I think this needs to be mentioned in some form at least.

Request URL : https://github.com/alsa-project/alsa-lib/pull/381 Patch URL : https://github.com/alsa-project/alsa-lib/pull/381.patch Repository URL: https://github.com/alsa-project/alsa-lib