Documentation/auxiliary_bus: Clarify __auxiliary_driver_register

__auxiliary_driver_register is not intended to be called directly unless
a custom name is required.  Add documentation for this fact.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20211202044305.4006853-5-ira.weiny@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>