ALSA: pcm: Comment why read blocks when PCM is not running

This avoids bringing back the problem introduced by
62ba568f ("ALSA: pcm: Return 0 when size <
start_threshold in capture") and fixed in 00a399ca


("ALSA: pcm: Revert capture stream behavior change in
blocking mode"), which prevented the user from starting
capture from another thread.

Signed-off-by: Ricardo Biehl Pasquali <pasqualirb@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>