Docs/admin-guide/mm/damon/usage: document effective_bytes file

Update DAMON usage document for the effective quota file of the DAMON
sysfs interface.

Link: https://lkml.kernel.org/r/20240219194431.159606-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>