ALSA: dummy - Add more description

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

Documentation/sound/alsa/ALSA-Configuration.txt

@@ -513,6 +513,19 @@ Prior to version 0.9.0rc4 options had a 'snd_' prefix. This was removed.
     or input, but you may use this module for any application which
     requires a sound card (like RealPlayer).
 
+    pcm_devs       - Number of PCM devices assigned to each card
+                     (default = 1, up to 4)
+    pcm_substreams - Number of PCM substreams assigned to each PCM
+                     (default = 8, up to 16)
+    hrtimer        - Use hrtimer (=1, default) or system timer (=0)
+
+    When multiple PCM devices are created, snd-dummy gives different
+    behavior to each PCM device:
+      0 = interleaved with mmap support
+      1 = non-interleaved with mmap support
+      2 = interleaved without mmap 
+      3 = non-interleaved without mmap
+
     The power-management is supported.
 
   Module snd-echo3g