Revert "ACPI: Documentation: driver-api: Disapprove of using ACPI drivers"

Revert commit b8c8a8ea18ad ("ACPI: Documentation: driver-api: Disapprove
of using ACPI drivers") because the document added by it is not
necessary any more after eliminating struct acpi_driver from the code.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>