Documentation: document debugpat commandline option

Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

Documentation/kernel-parameters.txt

@@ -571,6 +571,8 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	debug_objects	[KNL] Enable object debugging
 
+	debugpat	[X86] Enable PAT debugging
+
 	decnet.addr=	[HW,NET]
 			Format: <area>[,<node>]
 			See also Documentation/networking/decnet.txt.