USB: update documentation for usbmon

USB: update documentation for usbmon

commit d8cae98cddd286e38db1724dda1b0e7b467f9237 upstream.

The documentation for usbmon is out of date; the usbfs "devices" file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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>