ALSA: docs: Add description about ack callback -EPIPE error handling

Add a brief description about the newly added behavior of the PCM ack
callback with -EPIPE error.

Link: https://lore.kernel.org/r/20230323065237.5062-3-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>