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>

Author: nhatsmrt

Documentation/mm/damon/design.rst

@@ -656,6 +656,8 @@ Below ``type`` of filters are currently supported.
 - Operations layer handled, supported by only ``paddr`` operations set.
     - anon
         - Applied to pages that containing data that not stored in files.
+    - active
+        - Applied to active pages.
     - memcg
         - Applied to pages that belonging to a given cgroup.
     - young