documentation: describe bootmem_debug kernel parameter

"bootmem_debug" is not mentioned in kernel-parameters.txt. Recently I
had to use that kernel option and I think it should be documented.
Signed-off-by: NAndreas Herrmann <andreas.herrmann3@amd.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/kernel-parameters.txt

@@ -365,6 +365,8 @@ and is between 256 and 4096 characters. It is defined in the file
 			no delay (0).
 			Format: integer
 
+	bootmem_debug	[KNL] Enable bootmem allocator debug messages.
+
 	bttv.card=	[HW,V4L] bttv (bt848 + bt878 based grabber cards)
 	bttv.radio=	Most important insmod options are available as
 			kernel args too.