Docs/admin-guide/mm/damon/usage: introduce DAMON modules at the beginning

DAMON usage document provides a list of available DAMON interfaces with
brief introduction at the beginning of the doc.  The list is missing DAMON
modules for special purposes, while it is one of the major suggested
interfaces.  Add an item for those to the list.

Link: https://lkml.kernel.org/r/20260118180305.70023-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/mm/damon/usage.rst

@@ -6,6 +6,11 @@ Detailed Usages
 
 DAMON provides below interfaces for different users.
 
+- *Special-purpose DAMON modules.*
+  :ref:`This <damon_modules_special_purpose>` is for people who are building,
+  distributing, and/or administrating the kernel with special-purpose DAMON
+  usages.  Using this, users can use DAMON's major features for the given
+  purposes in build, boot, or runtime in simple ways.
 - *DAMON user space tool.*
   `This <https://github.com/damonitor/damo>`_ is for privileged people such as
   system administrators who want a just-working human-friendly interface.