USB: usbmon doc update - mention new wildcard ('0') bus

Update usbmon documentation, mentioning the "zero" (wildcard) bus.
Possibly, in my first hunk, the 'either ... or ...' should be rephrased a bit to
be expressed better.
Signed-off-by: NPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/usb/usbmon.txt

@@ -34,9 +34,12 @@ if usbmon is built into the kernel.
 Verify that bus sockets are present.
 
 # ls /sys/kernel/debug/usbmon
-1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u
+0s  0t  0u  1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u
 #
 
+Now you can choose to either use the sockets numbered '0' (to capture packets on
+all buses), and skip to step #3, or find the bus used by your device with step #2.
+
 2. Find which bus connects to the desired device
 
 Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
@@ -56,6 +59,10 @@ Bus=03 means it's bus 3.
 
 # cat /sys/kernel/debug/usbmon/3u > /tmp/1.mon.out
 
+to listen on a single bus, otherwise, to listen on all buses, type:
+
+# cat /sys/kernel/debug/usbmon/0u > /tmp/1.mon.out
+
 This process will be reading until killed. Naturally, the output can be
 redirected to a desirable location. This is preferred, because it is going
 to be quite long.