Docs/mm/damon/design: document unmapped DAMOS filter type

Document availability and meaning of unmapped 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/20250219220146.133650-3-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>