Docs/mm/damon/design: add a section for the relation between Core and Modules layer

Add overall desription of the interface and the relation between the Core
and the Modules layer under 'Overall Architecture' section.

Link: https://lkml.kernel.org/r/20230525214314.5204-7-sj@kernel.orgSigned-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

@@ -41,6 +41,16 @@ Also, if some architectures or devices supporting special optimized access
 check primitives, those will be easily configurable.
 
 
+Programmable Modules
+--------------------
+
+Core layer of DAMON is implemented as a framework, and exposes its application
+programming interface to all kernel space components such as subsystems and
+modules.  For common use cases of DAMON, DAMON subsystem provides kernel
+modules that built on top of the core layer using the API, which can be easily
+used by the user space end users.
+
+
 Operations Set Layer
 ====================