Docs/mm/damon/design: document DAMOS regions walking

DAMOS' regions walking is a feature for efficiently retrieving monitoring
results or DAMOS-internal behavior.  It can be useful for multiple
purposes including investigations and tuning.  Add a section for it on the
design document.

Link: https://lkml.kernel.org/r/20250103174400.54890-9-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>