thinkpad-acpi: document backlight level writeback at driver init

Document this, it is no fun to try to second guess why this sort of
stuff is in place years after it was added...
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>

drivers/platform/x86/thinkpad_acpi.c

@@ -6397,6 +6397,10 @@ static int __init brightness_init(struct ibm_init_struct *iibm)
 			"or not on your ThinkPad\n", TPACPI_MAIL);
 	}
 
+	/* Added by mistake in early 2007.  Probably useless, but it could
+	 * be working around some unknown firmware problem where the value
+	 * read at startup doesn't match the real hardware state... so leave
+	 * it in place just in case */
 	backlight_update_status(ibm_backlight_device);
 
 	vdbg_printk(TPACPI_DBG_INIT | TPACPI_DBG_BRGHT,