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 <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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>