[ACPI] update hotkey documentation

http://bugzilla.kernel.org/show_bug.cgi?id=4903

Signed-off-by: Iacopo Spalletti <avvisi@spalletti.it>
Signed-off-by: Len Brown <len.brown@intel.com>

Documentation/acpi-hotkey.txt

@@ -33,3 +33,6 @@ The result of the execution of this aml method is
 attached to /proc/acpi/hotkey/poll_method, which is dnyamically
 created.  Please use command "cat /proc/acpi/hotkey/polling_method" 
 to retrieve it.
+
+Note: Use cmdline "acpi_specific_hotkey" to enable legacy platform
+specific drivers.