Docs/mm/damon: move monitoring target regions setup detail from the usage to the design document

Design doc is aimed to have all concept level details, while the usage doc
is focused on only how the features can be used.  Some details about
monitoring target regions construction is on the usage doc.  Move the
details about the monitoring target regions construction differences for
DAMON operations set from the usage to the design doc.

Link: https://lkml.kernel.org/r/20240217005842.87348-5-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>