ALSA: hda - Add description of beep_mode in ALSA-Configuration.txt

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

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -798,6 +798,9 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
     		setup before initializing the codecs.  This option is
 		available only when CONFIG_SND_HDA_PATCH_LOADER=y is set.
 		See HD-Audio.txt for details.
+    beep_mode	- Selects the beep registration mode (0=off, 1=on, 2=
+		dynamic registration via mute switch on/off); the default
+		value is set via CONFIG_SND_HDA_INPUT_BEEP_MODE kconfig.
     
     [Single (global) options]
     single_cmd  - Use single immediate commands to communicate with