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>

Documentation/mm/damon/design.rst

@@ -540,6 +540,17 @@ To know how user-space can set the watermarks via :ref:`DAMON sysfs interface
 documentation.
 
 
+Regions Walking
+~~~~~~~~~~~~~~~
+
+DAMOS feature allowing users access each region that a DAMOS action has just
+applied.  Using this feature, DAMON :ref:`API <damon_design_api>` allows users
+access full properties of the regions including the access monitoring results.
+:ref:`DAMON sysfs interface <sysfs_interface>` also allows users read the data
+via special :ref:`files <sysfs_schemes_tried_regions>`.
+
+.. _damon_design_api:
+
 Application Programming Interface
 ---------------------------------