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

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -2137,6 +2137,10 @@ This callback is also not mandatory. This callback is called when the
 emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
 internal buffer, and this callback is useful only for such a purpose.
 
+The callback function may return 0 or a negative error. When the
+return value is ``-EPIPE``, PCM core treats as a buffer XRUN happens,
+and changes the state to ``SNDRV_PCM_STATE_XRUN`` automatically.
+
 This callback is atomic as default.
 
 page callback