ACPI: document acpi_sleep option

Index: linux.new/Documentation/kernel-parameters.txt
===================================================================
@@ -90,6 +90,10 @@
 			off -- disabled ACPI for systems with default on
 			ht -- run only enough ACPI to enable Hyper Threading
 			See also Documentation/pm.txt.
+
+	acpi_sleep=	[HW,ACPI] Sleep options
+			Format: { s3_bios, s3_mode }
+			See Documentation/power/video.txt
  
 	ad1816=		[HW,OSS]
 			Format: <io>,<irq>,<dma>,<dma2>