Docs/admin-guide/mm/damon/usage: link design document for DAMOS

The background and concept of DAMOS is redundantly documented, in the
design document and the usage document.  Replace the duplicated ones in
usage document with links to the design document.

Link: https://lkml.kernel.org/r/20230616191742.87531-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>