ALSA: Add missing description of usb-audio parameters

Added the missing description of module parameters of usb-audio driver
to ALSA-Configuration.txt.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@perex.cz>

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -1978,12 +1978,20 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
 
     vid             - Vendor ID for the device (optional)
     pid             - Product ID for the device (optional)
+    nrpacks	    - Max. number of packets per URB (default: 8)
+    async_unlink    - Use async unlink mode (default: yes)
     device_setup    - Device specific magic number (optional)
                     - Influence depends on the device
                     - Default: 0x0000 
 
     This module supports multiple devices, autoprobe and hotplugging.
 
+    NB: nrpacks parameter can be modified dynamically via sysfs.
+        Don't put the value over 20.  Changing via sysfs has no sanity
+	check.
+    NB: async_unlink=0 would cause Oops.  It remains just for
+        debugging purpose (if any).
+
   Module snd-usb-caiaq
   --------------------