USB: update documentation for usbmon

The documentation for usbmon is out of date; the usbfs "devices" file
now exists in /sys/kernel/debug/usb rather than /proc/bus/usb.  This
patch (as1505) updates the documentation accordingly, and also
mentions that the necessary information can be found by running lsusb.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Pete Zaitcev <zaitcev@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>