Document lowmem_reserve_ratio

Though the lower_zone_protection was changed to lowmem_reserve_ratio, the
document has been not changed.  The lowmem_reserve_ratio seems quite hard
to estimate, but there is no guidance.  This patch is to change document
for it.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Andrea Arcangeli <andrea@cpushare.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>