Document powerpc udbg-immortal

Back in 2006 the "udbg-immortal" kernel option has been introduced:

  > commit 3b5e905e
  > Author: Benjamin Herrenschmidt <benh@au1.ibm.com>
  > Date:   Wed Jun 7 12:06:20 2006 +1000
  >
  >  [PATCH] powerpc: Add udbg-immortal kernel option

...but I could not find it documented anywhere in the sources.
This patch adds it to Documentation/kernel-parameters.txt.
Signed-off-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/kernel-parameters.txt

@@ -2524,6 +2524,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			<port#>,<js1>,<js2>,<js3>,<js4>,<js5>,<js6>,<js7>
 			See also Documentation/input/joystick-parport.txt
 
+	udbg-immortal	[PPC] When debugging early kernel crashes that
+			happen after console_init() and before a proper 
+			console driver takes over, this boot options might
+			help "seeing" what's going on.
+
 	uhash_entries=	[KNL,NET]
 			Set number of hash buckets for UDP/UDP-Lite connections