[ALSA] Add description about spdif_aclink option for snd-intel8x0

Added a description about spdif_aclink option for snd-intel8x0 driver
in ALSA-Configuration.txt.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -1079,6 +1079,9 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
     buggy_semaphore - Enable workaround for hardwares with buggy
 		    semaphores (e.g. on some ASUS laptops)
 		    (default off)
+    spdif_aclink  - Use S/PDIF over AC-link instead of direct connection
+		    from the controller chip
+		    (0 = off, 1 = on, -1 = default)
 
     This module supports one chip and autoprobe.