[ALSA] ad1889: add AD1889 driver docs

Documentation
move the AD1889 driver docs to the kernel tree, too
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -148,6 +148,16 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
     Module supports up to 8 cards. This module does not support autoprobe
     thus main port must be specified!!! Other ports are optional.
     
+  Module snd-ad1889
+  -----------------
+
+    Module for Analog Devices AD1889 chips.
+
+    ac97_quirk  - AC'97 workaround for strange hardware
+                  See the description of intel8x0 module for details.
+
+    This module supports up to 8 cards.
+
   Module snd-ali5451
   ------------------