Docs/mm/damon/design: add 'statistics' section

DAMOS stats are important feature for tuning of DAMOS-based access-aware
system operation, and efficient access pattern monitoring.  But not well
documented on the design document.  Add a section on the document.

Link: https://lkml.kernel.org/r/20250106193401.109161-3-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>