Docs/admin-guide/mm/damon/usage: document 'DEPRECATED' file of DAMON debugfs interface

Document the newly added DAMON debugfs interface deprecation notice file
on the usage document.

Link: https://lkml.kernel.org/r/20240130013549.89538-6-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

@@ -628,9 +628,16 @@ debugfs Interface (DEPRECATED!)
   move, please report your usecase to damon@lists.linux.dev and
   linux-mm@kvack.org.
 
-DAMON exports eight files, ``attrs``, ``target_ids``, ``init_regions``,
-``schemes``, ``monitor_on``, ``kdamond_pid``, ``mk_contexts`` and
-``rm_contexts`` under its debugfs directory, ``<debugfs>/damon/``.
+DAMON exports nine files, ``DEPRECATED``, ``attrs``, ``target_ids``,
+``init_regions``, ``schemes``, ``monitor_on``, ``kdamond_pid``, ``mk_contexts``
+and ``rm_contexts`` under its debugfs directory, ``<debugfs>/damon/``.
+
+
+``DEPRECATED`` is a read-only file for the DAMON debugfs interface deprecation
+notice.  Reading it returns the deprecation notice, as below::
+
+    # cat DEPRECATED
+    DAMON debugfs interface is deprecated, so users should move to DAMON_SYSFS. If you cannot, please report your usecase to damon@lists.linux.dev and linux-mm@kvack.org.
 
 
 Attributes