mm: add a reminder comment for __GFP_BITS_SHIFT

Cc: Glauber Costa <glommer@parallels.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>