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: Andreas Herrmann <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/kernel-parameters.txt

···
 			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.