Docs/admin-guide/mm/damon/usage: note DAMON debugfs interface deprecation plan

Docs/admin-guide/mm/damon/usage: note DAMON debugfs interface deprecation plan

Commit b18402726bd1 ("Docs/admin-guide/mm/damon/usage: document DAMON
sysfs interface") announced the DAMON debugfs interface deprecation plan,
but it is not so aggressively announced.  As the deprecation time is
coming, this commit makes the announce more easy to be found by adding the
note at the beginning of the DAMON debugfs interface usage document.

Link: https://lkml.kernel.org/r/20220909202901.57977-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yun Levi <ppbuk5246@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>