ALSA: pcm: add comment about application of rule to PCM parameters

Drivers add rules of parameters to runtime of PCM substream, when
applications open ALSA PCM character device. When applications call
ioctl(2) with SNDRV_PCM_IOCTL_HW_REFINE or SNDRV_PCM_IOCTL_HW_PARAMS, the
rules are applied to the parameters and return the result to user space.

The rule can have dependency between parameters. Additionally, it can have
condition flags about application of rules. Userspace applications can
indicate the flags to suppress change of parameters.

This commit attempts to describe the mechanism.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>