ALSA: doc: Update the description about PCM suspend procedure

The PCM suspend procedure was changed for drivers, so that they don't
have to call snd_pcm_suspend*() in each callback any longer.  Update
the documentation to adapt the changes.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>