ACPI: Improve acpi debug documentation

Now we use acpi.debug_level and acpi.debug_layer as kernel boot
parameters instead of acpi_dbg_level and acpi_dbg_layer.
Thanks to Andi Kleen for pointing it out.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>