ABI: sysfs-bus-usb: better document variable argument

ABI: sysfs-bus-usb: better document variable argument

On almost all ABI documents, variable arguments are declared
as <foo_bar>. Change it here too, in order to allow replacing
such wildcards by regexes on a scriptable way.

Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Acked-by: Rajat Jain <rajatja@google.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/2f7e4e874677dbd82693a6b219decefa18802e8f.1631782432.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>