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: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>