ALSA: Update documents about new bits of xrun_debug proc file

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

Documentation/sound/alsa/Procfile.txt

@@ -103,6 +103,8 @@ card*/pcm*/xrun_debug
 	  bit 2 = Enable additional jiffies check
 	  bit 3 = Log hwptr update at each period interrupt
 	  bit 4 = Log hwptr update at each snd_pcm_update_hw_ptr()
+	  bit 5 = Show last 10 positions on error
+	  bit 6 = Do above only once
 
 	When the bit 0 is set, the driver will show the messages to
 	kernel log when an xrun is detected.  The debug message is
@@ -122,6 +124,12 @@ card*/pcm*/xrun_debug
 	Bits 3 and 4 are for logging the hwptr records.  Note that
 	these will give flood of kernel messages.
 
+	When bit 5 is set, the driver logs the last 10 xrun errors and
+	the proc file shows each jiffies, position, period_size,
+	buffer_size, old_hw_ptr, and hw_ptr_base values.
+
+	When bit 6 is set, the full xrun log is shown only once.
+
 card*/pcm*/sub*/info
 	The general information of this PCM sub-stream.