Docs/mm/damon/design: add a section for kdamond and DAMON context

The design document is not explaining about the concept of kdamond and
the DAMON context, while usage document does.  Those concept explanation
should be in the design document, and usage document should link those.
Add a section for those.

Link: https://lkml.kernel.org/r/20230907022929.91361-7-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/damon/design.rst

@@ -476,3 +476,13 @@ modules for proactive reclamation and LRU lists manipulation are provided.  For
 more detail, please read the usage documents for those
 (:doc:`/admin-guide/mm/damon/reclaim` and
 :doc:`/admin-guide/mm/damon/lru_sort`).
+
+
+Execution Model and Data Structures
+===================================
+
+The monitoring-related information including the monitoring request
+specification and DAMON-based operation schemes are stored in a data structure
+called DAMON ``context``.  DAMON executes each context with a kernel thread
+called ``kdamond``.  Multiple kdamonds could run in parallel, for different
+types of monitoring.