docs/mm/damon/design: document active DAMOS filter type

Document availability and meaning of "active" DAMOS filter type on design
document.  Since introduction of the type requires no additional user ABI,
usage and ABI document need no update.

Link: https://lkml.kernel.org/r/20250318183029.2062917-3-nphamcs@gmail.com


Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>